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JVM运行时参数

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2025-07-21 19:17:07

面试题

JVM的参数，你知道的说一下 (百度) 说说你知道的几种主要的JVM参数（京东） JVM调优调的哪些参数？在哪里写这些参数？（亚信）内存调优参数都有什么？（顺丰） JVM调优调的哪些参数？问这样有什么好处？在哪里写这些参数（浪潮）

JVM参数选项类型

官网说明： docs.oracle /javase/8/docs/technotes/tools/unix/java.html

类型一：标准参数选项

特点：

比较稳定，后续版本基本不会变化；以-开头

各种选项：

运行java或者java -help可以看到所有的标准选项

-d32 使用 32 位数据模型 (如果可用)

-d64 使用 64 位数据模型 (如果可用)

-server 选择 "server" VM

默认 VM 是 server.

-cp <目录和 zip/jar 文件的类搜索路径>

-classpath <目录和 zip/jar 文件的类搜索路径>

用 ; 分隔的目录, JAR 档案

和 ZIP 档案列表, 用于搜索类文件。

-D<名称>=<值>

设置系统属性

-verbose:[class|gc|jni]

启用详细输出

-version 输出产品版本并退出

-version:<值>

警告: 此功能已过时, 将在

未来发行版中删除。

需要指定的版本才能运行

-showversion 输出产品版本并继续

-jre-restrict-search | -no-jre-restrict-search

警告: 此功能已过时, 将在

未来发行版中删除。

在版本搜索中包括/排除用户专用 JRE

-? -help 输出此帮助消息

-X 输出非标准选项的帮助

-ea[:<packagename>...|:<classname>]

-enableassertions[:<packagename>...|:<classname>]

按指定的粒度启用断言

-da[:<packagename>...|:<classname>]

-disableassertions[:<packagename>...|:<classname>]

禁用具有指定粒度的断言

-esa | -enablesystemassertions

启用系统断言

-dsa | -disablesystemassertions

禁用系统断言

-agentlib:<libname>[=<选项>]

加载本机代理库 <libname>, 例如 -agentlib:hprof

另请参阅 -agentlib:jdwp=help 和 -agentlib:hprof=help

-agentpath:<pathname>[=<选项>]

按完整路径名加载本机代理库

-javaagent:<jarpath>[=<选项>]

加载 Java 编程语言代理, 请参阅 java.lang.instrument

-splash:<imagepath>

使用指定的图像显示启动屏幕

有关详细信息, 请参阅：Java SE - Documentation

补充内容：-server与-client

Hotspot JVM有两种模式，分别是server和client，分别通过-server和-client模式设置

在32位Windows系统上，默认使用Client类型的JVM。要想使用Server模式，则机器配置至少有2个以上的CPU和2G以上的物理内存。 client模式适用于对内存要求较小的桌面应用程序，默认使用Serial串行垃圾收集器。

64位机器上只支持server模式的JVM，适用于需要大内存的应用程序，默认使用并行垃圾收集器。

关于server和client的官网介绍为： docs.oracle /javase/8/docs/technotes/guides/vm/server-class.html

类型二：-X参数选项

特点

非标准化参数；功能还是比较稳定的。但官方说后续版本可能会变更；以-X开头

各种选项

-Xmixed 混合模式执行 (默认)

-Xint 仅解释模式执行

-Xcomp 仅采用即时编译器模式

-Xbootclasspath:<用 ; 分隔的目录和 zip/jar 文件>

设置搜索路径以引导类和资源

-Xbootclasspath/a:<用 ; 分隔的目录和 zip/jar 文件>

附加在引导类路径末尾

-Xbootclasspath/p:<用 ; 分隔的目录和 zip/jar 文件>

置于引导类路径之前

-Xdiag 显示附加诊断消息

-Xnoclassgc 禁用类垃圾收集

-Xincgc 启用增量垃圾收集

-Xloggc:<file> 将 GC 状态记录在文件中 (带时间戳)

-Xbatch 禁用后台编译

-Xms<size> 设置初始 Java 堆大小

-Xmx<size> 设置最大 Java 堆大小

-Xss<size> 设置 Java 线程堆栈大小

-Xprof 输出 cpu 配置文件数据

-Xfuture 启用最严格的检查, 预期将来的默认值

-Xrs 减少 Java/VM 对操作系统信号的使用 (请参阅文档)

-Xcheck:jni 对 JNI 函数执行其他检查

-Xshare:off 不尝试使用共享类数据

-Xshare:auto 在可能的情况下使用共享类数据 (默认)

-Xshare:on 要求使用共享类数据, 否则将失败。

-XshowSettings 显示所有设置并继续

-XshowSettings:all 显示所有设置并继续

-XshowSettings:vm 显示所有与 vm 相关的设置并继续

-XshowSettings:properties 显示所有属性设置并继续

-XshowSettings:locale 显示所有与区域设置相关的设置并继续

-X 选项是非标准选项, 如有更改, 恕不另行通知。

JVM的JIT编译模式相关的选项

-Xint

禁用JIT，所有字节码都被解释执行，这个模式的速度最慢的

-Xcomp

所有字节码第一次使用就都被编译成本地代码，然后再执行

-Xmixed

混合模式，默认模式，让JIT根据程序运行的情况，有选择地将某些代码编译成本地代码

特别地

-Xmx -Xms -Xss属于XX参数？

-Xms<size>

设置初始Java堆大小，等价于-XX:InitialHeapSize

-Xmx<size>

设置最大Java堆大小，等价于-XX:MaxHeapSize

-Xss<size>

设置Java线程堆栈大小，-XX:ThreadStackSize

类型三：-XX参数选项

特点

非标准化参数；使用的最多的参数类型；这类选项属于实验性，不稳定；以-XX开头

作用

用于开发和调试JVM

分类

1、Boolean类型格式

-XX:+<option>表示启用option属性

-XX:-<option>表示禁用option属性

举例：

-XX:-UseParallelGC 不选择垃圾收集器为并行收集器

-XX:+UseG1GC 表示启用G1收集器

-XX:+UseAdaptiveSizePolicy 自动选择年轻代区大小和相应的Survivor区比例

说明：因为有的指令默认是开启的，所以可以-使用关闭

2、非Boolean类型格式(key-value类型)

子类型1：数值型格式-XX:<option>=<number>

number表示数值，number可以带上单位，比如：'m' 、'M' 表示兆， 'k' 、'K' 表示Kb， 'g' 、 'G' 表示 g（例如 32k跟32768是一样的效果）

例如：

-XX:NewSize=1024m 表示设置新生代初始大小为1024兆

-XX:MaxGCPauseMillis=500 表示设置GC停顿时间：500毫秒

-XX:GCTimeRatio=19 表示设置吞吐量

-XX:NewRatio=2 表示新生代与老年代的比例

子类型2：非数值型格式-XX:<name>=<string>

例如：-XX:HeapDumpPath=/usr/local/heapdump.hprof 用来指定heap转存文件的存储路径。

特别地

-XX:+PrintFlagsFinal

输出所有参数的名称和默认值

默认不包括Diagnostic和Experimental的参数

可以配合-XX:+UnlockDiagnosticVMOptions和-XX:UnlockExperimentalVMOptions使用

各种参数默认值（参考）

[Global flags] intx ActiveProcessorCount = -1 {product} uintx AdaptiveSizeDecrementScaleFactor = 4 {product} uintx AdaptiveSizeMajorGCDecayTimeScale = 10 {product} uintx AdaptiveSizePausePolicy = 0 {product} uintx AdaptiveSizePolicyCollectionCostMargin = 50 {product} uintx AdaptiveSizePolicyInitializingSteps = 20 {product} uintx AdaptiveSizePolicyOutputInterval = 0 {product} uintx AdaptiveSizePolicyWeight = 10 {product} uintx AdaptiveSizeThroughPutPolicy = 0 {product} uintx AdaptiveTimeWeight = 25 {product} bool AdjustConcurrency = false {product} bool AggressiveHeap = false {product} bool AggressiveOpts = false {product} intx AliasLevel = 3 {C2 product} bool AlignVector = true {C2 product} intx AllocateInstancePrefetchLines = 1 {product} intx AllocatePrefetchDistance = 256 {product} intx AllocatePrefetchInstr = 3 {product} intx AllocatePrefetchLines = 3 {product} intx AllocatePrefetchStepSize = 64 {product} intx AllocatePrefetchStyle = 1 {product} bool AllowJNIEnvProxy = false {product} bool AllowNonVirtualCalls = false {product} bool AllowParallelDefineClass = false {product} bool AllowUserSignalHandlers = false {product} bool AlwaysActAsServerClassMachine = false {product} bool AlwaysCompileLoopMethods = false {product} bool AlwaysLockClassLoader = false {product} bool AlwaysPreTouch = false {product} bool AlwaysRestoreFPU = false {product} bool AlwaysTenure = false {product} bool AssertOnSuspendWaitFailure = false {product} bool AssumeMP = false {product} intx AutoBoxCacheMax = 128 {C2 product} uintx AutoGCSelectPauseMillis = 5000 {product} intx BCEATraceLevel = 0 {product} intx BackEdgeThreshold = 100000 {pd product} bool BackgroundCompilation = true {pd product} uintx BaseFootPrintEstimate = 268435456 {product} intx BiasedLockingBulkRebiasThreshold = 20 {product} intx BiasedLockingBulkRevokeThreshold = 40 {product} intx BiasedLockingDecayTime = 25000 {product} intx BiasedLockingStartupDelay = 4000 {product} bool BindGCTaskThreadsToCPUs = false {product} bool BlockLayoutByFrequency = true {C2 product} intx BlockLayoutMinDiamondPercentage = 20 {C2 product} bool BlockLayoutRotateLoops = true {C2 product} bool BranchOnRegister = false {C2 product} bool BytecodeVerificationLocal = false {product} bool BytecodeVerificationRemote = true {product} bool C1OptimizeVirtualCallProfiling = true {C1 product} bool C1ProfileBranches = true {C1 product} bool C1ProfileCalls = true {C1 product} bool C1ProfileCheckcasts = true {C1 product} bool C1ProfileInlinedCalls = true {C1 product} bool C1ProfileVirtualCalls = true {C1 product} bool C1UpdateMethodData = true {C1 product} intx CICompilerCount := 12 {product} bool CICompilerCountPerCPU = true {product} bool CITime = false {product} bool CMSAbortSemantics = false {product} uintx CMSAbortablePrecleanMinWorkPerIteration = 100 {product} intx CMSAbortablePrecleanWaitMillis = 100 {manageable} uintx CMSBitMapYieldQuantum = 10485760 {product} uintx CMSBootstrapOccupancy = 50 {product} bool CMSClassUnloadingEnabled = true {product} uintx CMSClassUnloadingMaxInterval = 0 {product} bool CMSCleanOnEnter = true {product} bool CMSCompactWhenClearAllSoftRefs = true {product} uintx CMSConcMarkMultiple = 32 {product} bool CMSConcurrentMTEnabled = true {product} uintx CMSCoordinatorYieldSleepCount = 10 {product} bool CMSDumpAtPromotionFailure = false {product} bool CMSEdenChunksRecordAlways = true {product} uintx CMSExpAvgFactor = 50 {product} bool CMSExtrapolateSweep = false {product} uintx CMSFullGCsBeforeCompaction = 0 {product} uintx CMSIncrementalDutyCycle = 10 {product} uintx CMSIncrementalDutyCycleMin = 0 {product} bool CMSIncrementalMode = false {product} uintx CMSIncrementalOffset = 0 {product} bool CMSIncrementalPacing = true {product} uintx CMSIncrementalSafetyFactor = 10 {product} uintx CMSIndexedFreeListReplenish = 4 {product} intx CMSInitiatingOccupancyFraction = -1 {product} uintx CMSIsTooFullPercentage = 98 {product} double CMSLargeCoalSurplusPercent = 0.950000 {product} double CMSLargeSplitSurplusPercent = 1.000000 {product} bool CMSLoopWarn = false {product} uintx CMSMaxAbortablePrecleanLoops = 0 {product} intx CMSMaxAbortablePrecleanTime = 5000 {product} uintx CMSOldPLABMax = 1024 {product} uintx CMSOldPLABMin = 16 {product} uintx CMSOldPLABNumRefills = 4 {product} uintx CMSOldPLABReactivityFactor = 2 {product} bool CMSOldPLABResizeQuicker = false {product} uintx CMSOldPLABToleranceFactor = 4 {product} bool CMSPLABRecordAlways = true {product} uintx CMSParPromoteBlocksToClaim = 16 {product} bool CMSParallelInitialMarkEnabled = true {product} bool CMSParallelRemarkEnabled = true {product} bool CMSParallelSurvivorRemarkEnabled = true {product} uintx CMSPrecleanDenominator = 3 {product} uintx CMSPrecleanIter = 3 {product} uintx CMSPrecleanNumerator = 2 {product} bool CMSPrecleanRefLists1 = true {product} bool CMSPrecleanRefLists2 = false {product} bool CMSPrecleanSurvivors1 = false {product} bool CMSPrecleanSurvivors2 = true {product} uintx CMSPrecleanThreshold = 1000 {product} bool CMSPrecleaningEnabled = true {product} bool CMSPrintChunksInDump = false {product} bool CMSPrintEdenSurvivorChunks = false {product} bool CMSPrintObjectsInDump = false {product} uintx CMSRemarkVerifyVariant = 1 {product} bool CMSReplenishIntermediate = true {product} uintx CMSRescanMultiple = 32 {product} uintx CMSSamplingGrain = 16384 {product} bool CMSScavengeBeforeRemark = false {product} uintx CMSScheduleRemarkEdenPenetration = 50 {product} uintx CMSScheduleRemarkEdenSizeThreshold = 2097152 {product} uintx CMSScheduleRemarkSamplingRatio = 5 {product} double CMSSmallCoalSurplusPercent = 1.050000 {product} double CMSSmallSplitSurplusPercent = 1.100000 {product} bool CMSSplitIndexedFreeListBlocks = true {product} intx CMSTriggerInterval = -1 {manageable} uintx CMSTriggerRatio = 80 {product} intx CMSWaitDuration = 2000 {manageable} uintx CMSWorkQueueDrainThreshold = 10 {product} bool CMSYield = true {product} uintx CMSYieldSleepCount = 0 {product} uintx CMSYoungGenPerWorker = 67108864 {pd product} uintx CMS_FLSPadding = 1 {product} uintx CMS_FLSWeight = 75 {product} uintx CMS_SweepPadding = 1 {product} uintx CMS_SweepTimerThresholdMillis = 10 {product} uintx CMS_SweepWeight = 75 {product} bool CheckEndorsedAndExtDirs = false {product} bool CheckJNICalls = false {product} bool ClassUnloading = true {product} bool ClassUnloadingWithConcurrentMark = true {product} intx ClearFPUAtPark = 0 {product} bool ClipInlining = true {product} uintx CodeCacheExpansionSize = 65536 {pd product} uintx CodeCacheMinimumFreeSpace = 512000 {product} bool CollectGen0First = false {product} bool CompactFields = true {product} intx CompilationPolicyChoice = 3 {product} ccstrlist CompileCommand = {product} ccstr CompileCommandFile = {product} ccstrlist CompileOnly = {product} intx CompileThreshold = 10000 {pd product} bool CompilerThreadHintNoPreempt = true {product} intx CompilerThreadPriority = -1 {product} intx CompilerThreadStackSize = 0 {pd product} uintx CompressedClassSpaceSize = 1073741824 {product} uintx ConcGCThreads = 0 {product} intx ConditionalMoveLimit = 3 {C2 pd product} intx ContendedPaddingWidth = 128 {product} bool ConvertSleepToYield = true {pd product} bool ConvertYieldToSleep = false {product} bool CrashOnOutOfMemoryError = false {product} bool CreateMinidumpOnCrash = false {product} bool CriticalJNINatives = true {product} bool DTraceAllocProbes = false {product} bool DTraceMethodProbes = false {product} bool DTraceMonitorProbes = false {product} bool Debugging = false {product} uintx DefaultMaxRAMFraction = 4 {product} intx DefaultThreadPriority = -1 {product} intx DeferPollingPageLoopCount = -1 {product} intx DeferThrSuspendLoopCount = 4000 {product} bool DeoptimizeRandom = false {product} bool DisableAttachMechanism = false {product} bool DisableExplicitGC = false {product} bool DisplayVMOutputToStderr = false {product} bool DisplayVMOutputToStdout = false {product} bool DoEscapeAnalysis = true {C2 product} bool DontCompileHugeMethods = true {product} bool DontYieldALot = false {pd product} ccstr DumpLoadedClassList = {product} bool DumpReplayDataOnError = true {product} bool DumpSharedSpaces = false {product} bool EagerXrunInit = false {product} intx EliminateAllocationArraySizeLimit = 64 {C2 product} bool EliminateAllocations = true {C2 product} bool EliminateAutoBox = true {C2 product} bool EliminateLocks = true {C2 product} bool EliminateNestedLocks = true {C2 product} intx EmitSync = 0 {product} bool EnableContended = true {product} bool EnableResourceManagementTLABCache = true {product} bool EnableSharedLookupCache = true {product} bool EnableTracing = false {product} uintx ErgoHeapSizeLimit = 0 {product} ccstr ErrorFile = {product} ccstr ErrorReportServer = {product} double EscapeAnalysisTimeout = 20.000000 {C2 product} bool EstimateArgEscape = true {product} bool ExitOnOutOfMemoryError = false {product} bool ExplicitGCInvokesConcurrent = false {product} bool ExplicitGCInvokesConcurrentAndUnloadsClasses = false {product} bool ExtendedDTraceProbes = false {product} ccstr ExtraSharedClassListFile = {product} bool FLSAlwaysCoalesceLarge = false {product} uintx FLSCoalescePolicy = 2 {product} double FLSLargestBlockCoalesceProximity = 0.990000 {product} bool FailOverToOldVerifier = true {product} bool FastTLABRefill = true {product} intx FenceInstruction = 0 {ARCH product} intx FieldsAllocationStyle = 1 {product} bool FilterSpuriousWakeups = true {product} ccstr FlightRecorderOptions = {product} bool ForceNUMA = false {product} bool ForceTimeHighResolution = false {product} intx FreqInlineSize = 325 {pd product} double G1ConcMarkStepDurationMillis = 10.000000 {product} uintx G1ConcRSHotCardLimit = 4 {product} uintx G1ConcRSLogCacheSize = 10 {product} intx G1ConcRefinementGreenZone = 0 {product} intx G1ConcRefinementRedZone = 0 {product} intx G1ConcRefinementServiceIntervalMillis = 300 {product} uintx G1ConcRefinementThreads = 0 {product} intx G1ConcRefinementThresholdStep = 0 {product} intx G1ConcRefinementYellowZone = 0 {product} uintx G1ConfidencePercent = 50 {product} uintx G1HeapRegionSize = 0 {product} uintx G1HeapWastePercent = 5 {product} uintx G1MixedGCCountTarget = 8 {product} intx G1RSetRegionEntries = 0 {product} uintx G1RSetScanBlockSize = 64 {product} intx G1RSetSparseRegionEntries = 0 {product} intx G1RSetUpdatingPauseTimePercent = 10 {product} intx G1RefProcDrainInterval = 10 {product} uintx G1ReservePercent = 10 {product} uintx G1SATBBufferEnqueueingThresholdPercent = 60 {product} intx G1SATBBufferSize = 1024 {product} intx G1UpdateBufferSize = 256 {product} bool G1UseAdaptiveConcRefinement = true {product} uintx GCDrainStackTargetSize = 64 {product} uintx GCHeapFreeLimit = 2 {product} uintx GCLockerEdenExpansionPercent = 5 {product} bool GCLockerInvokesConcurrent = false {product} uintx GCLogFileSize = 8192 {product} uintx GCPauseIntervalMillis = 0 {product} uintx GCTaskTimeStampEntries = 200 {product} uintx GCTimeLimit = 98 {product} uintx GCTimeRatio = 99 {product} uintx HeapBaseMinAddress = 2147483648 {pd product} bool HeapDumpAfterFullGC = false {manageable} bool HeapDumpBeforeFullGC = false {manageable} bool HeapDumpOnOutOfMemoryError = false {manageable} ccstr HeapDumpPath = {manageable} uintx HeapFirstMaximumCompactionCount = 3 {product} uintx HeapMaximumCompactionInterval = 20 {product} uintx HeapSizePerGCThread = 87241520 {product} bool IgnoreEmptyClassPaths = false {product} bool IgnoreUnrecognizedVMOptions = false {product} uintx IncreaseFirstTierCompileThresholdAt = 50 {product} bool IncrementalInline = true {C2 product} uintx InitialBootClassLoaderMetaspaceSize = 4194304 {product} uintx InitialCodeCacheSize = 2555904 {pd product} uintx InitialHeapSize := 526385152 {product} uintx InitialRAMFraction = 64 {product} double InitialRAMPercentage = 1.562500 {product} uintx InitialSurvivorRatio = 8 {product} uintx InitialTenuringThreshold = 7 {product} uintx InitiatingHeapOccupancyPercent = 45 {product} bool Inline = true {product} ccstr InlineDataFile = {product} intx InlineSmallCode = 2000 {pd product} bool InlineSynchronizedMethods = true {C1 product} bool InsertMemBarAfterArraycopy = true {C2 product} intx InteriorEntryAlignment = 16 {C2 pd product} intx InterpreterProfilePercentage = 33 {product} bool JNIDetachReleasesMonitors = true {product} bool JavaMonitorsInStackTrace = true {product} intx JavaPriority10_To_OSPriority = -1 {product} intx JavaPriority1_To_OSPriority = -1 {product} intx JavaPriority2_To_OSPriority = -1 {product} intx JavaPriority3_To_OSPriority = -1 {product} intx JavaPriority4_To_OSPriority = -1 {product} intx JavaPriority5_To_OSPriority = -1 {product} intx JavaPriority6_To_OSPriority = -1 {product} intx JavaPriority7_To_OSPriority = -1 {product} intx JavaPriority8_To_OSPriority = -1 {product} intx JavaPriority9_To_OSPriority = -1 {product} bool LIRFillDelaySlots = false {C1 pd product} uintx LargePageHeapSizeThreshold = 134217728 {product} uintx LargePageSizeInBytes = 0 {product} bool LazyBootClassLoader = true {product} intx LiveNodeCountInliningCutoff = 40000 {C2 product} bool LoadExecStackDllInVMThread = true {product} bool LogCommercialFeatures = false {product} intx LoopMaxUnroll = 16 {C2 product} intx LoopOptsCount = 43 {C2 product} intx LoopUnrollLimit = 60 {C2 pd product} intx LoopUnrollMin = 4 {C2 product} bool LoopUnswitching = true {C2 product} bool ManagementServer = false {product} uintx MarkStackSize = 4194304 {product} uintx MarkStackSizeMax = 536870912 {product} uintx MarkSweepAlwaysCompactCount = 4 {product} uintx MarkSweepDeadRatio = 1 {product} intx MaxBCEAEstimateLevel = 5 {product} intx MaxBCEAEstimateSize = 150 {product} uintx MaxDirectMemorySize = 0 {product} bool MaxFDLimit = true {product} uintx MaxGCMinorPauseMillis = 18446744073709551615 {product} uintx MaxGCPauseMillis = 18446744073709551615 {product} uintx MaxHeapFreeRatio = 100 {manageable} uintx MaxHeapSize := 8417968128 {product} intx MaxInlineLevel = 9 {product} intx MaxInlineSize = 35 {product} intx MaxJNILocalCapacity = 65536 {product} intx MaxJavaStackTraceDepth = 1024 {product} intx MaxJumpTableSize = 65000 {C2 product} intx MaxJumpTableSparseness = 5 {C2 product} intx MaxLabelRootDepth = 1100 {C2 product} intx MaxLoopPad = 11 {C2 product} uintx MaxMetaspaceExpansion = 5451776 {product} uintx MaxMetaspaceFreeRatio = 70 {product} uintx MaxMetaspaceSize = 18446744073709547520 {product} uintx MaxNewSize := 2805989376 {product} intx MaxNodeLimit = 75000 {C2 product} uint64_t MaxRAM = 137438953472 {pd product} uintx MaxRAMFraction = 4 {product} double MaxRAMPercentage = 25.000000 {product} intx MaxRecursiveInlineLevel = 1 {product} uintx MaxTenuringThreshold = 15 {product} intx MaxTrivialSize = 6 {product} intx MaxVectorSize = 32 {C2 product} uintx MetaspaceSize = 21807104 {pd product} bool MethodFlushing = true {product} uintx MinHeapDeltaBytes := 524288 {product} uintx MinHeapFreeRatio = 0 {manageable} intx MinInliningThreshold = 250 {product} intx MinJumpTableSize = 10 {C2 pd product} uintx MinMetaspaceExpansion = 339968 {product} uintx MinMetaspaceFreeRatio = 40 {product} uintx MinRAMFraction = 2 {product} double MinRAMPercentage = 50.000000 {product} uintx MinSurvivorRatio = 3 {product} uintx MinTLABSize = 2048 {product} intx MonitorBound = 0 {product} bool MonitorInUseLists = false {product} intx MultiArrayExpandLimit = 6 {C2 product} bool MustCallLoadClassInternal = false {product} uintx NUMAChunkResizeWeight = 20 {product} uintx NUMAInterleaveGranularity = 2097152 {product} uintx NUMAPageScanRate = 256 {product} uintx NUMASpaceResizeRate = 1073741824 {product} bool NUMAStats = false {product} ccstr NativeMemoryTracking = off {product} bool NeedsDeoptSuspend = false {pd product} bool NeverActAsServerClassMachine = false {pd product} bool NeverTenure = false {product} uintx NewRatio = 2 {product} uintx NewSize := 175112192 {product} uintx NewSizeThreadIncrease = 5320 {pd product} intx NmethodSweepActivity = 10 {product} intx NmethodSweepCheckInterval = 5 {product} intx NmethodSweepFraction = 16 {product} intx NodeLimitFudgeFactor = 2000 {C2 product} uintx NumberOfGCLogFiles = 0 {product} intx NumberOfLoopInstrToAlign = 4 {C2 product} intx ObjectAlignmentInBytes = 8 {lp64_product} uintx OldPLABSize = 1024 {product} uintx OldPLABWeight = 50 {product} uintx OldSize := 351272960 {product} bool OmitStackTraceInFastThrow = true {product} ccstrlist OnError = {product} ccstrlist OnOutOfMemoryError = {product} intx OnStackReplacePercentage = 140 {pd product} bool OptimizeFill = true {C2 product} bool OptimizePtrCompare = true {C2 product} bool OptimizeStringConcat = true {C2 product} bool OptoBundling = false {C2 pd product} intx OptoLoopAlignment = 16 {pd product} bool OptoScheduling = false {C2 pd product} uintx PLABWeight = 75 {product} bool PSChunkLargeArrays = true {product} intx ParGCArrayScanChunk = 50 {product} uintx ParGCDesiredObjsFromOverflowList = 20 {product} bool ParGCTrimOverflow = true {product} bool ParGCUseLocalOverflow = false {product} uintx ParallelGCBufferWastePct = 10 {product} uintx ParallelGCThreads = 13 {product} bool ParallelGCVerbose = false {product} uintx ParallelOldDeadWoodLimiterMean = 50 {product} uintx ParallelOldDeadWoodLimiterStdDev = 80 {product} bool ParallelRefProcBalancingEnabled = true {product} bool ParallelRefProcEnabled = false {product} bool PartialPeelAtUnsignedTests = true {C2 product} bool PartialPeelLoop = true {C2 product} intx PartialPeelNewPhiDelta = 0 {C2 product} uintx PausePadding = 1 {product} intx PerBytecodeRecompilationCutoff = 200 {product} intx PerBytecodeTrapLimit = 4 {product} intx PerMethodRecompilationCutoff = 400 {product} intx PerMethodTrapLimit = 100 {product} bool PerfAllowAtExitRegistration = false {product} bool PerfBypassFileSystemCheck = false {product} intx PerfDataMemorySize = 32768 {product} intx PerfDataSamplingInterval = 50 {product} ccstr PerfDataSaveFile = {product} bool PerfDataSaveToFile = false {product} bool PerfDisableSharedMem = false {product} intx PerfMaxStringConstLength = 1024 {product} intx PreInflateSpin = 10 {pd product} bool PreferContainerQuotaForCPUCount = true {product} bool PreferInterpreterNativeStubs = false {pd product} intx PrefetchCopyIntervalInBytes = 576 {product} intx PrefetchFieldsAhead = 1 {product} intx PrefetchScanIntervalInBytes = 576 {product} bool PreserveAllAnnotations = false {product} bool PreserveFramePointer = false {pd product} uintx PretenureSizeThreshold = 0 {product} bool PrintAdaptiveSizePolicy = false {product} bool PrintCMSInitiationStatistics = false {product} intx PrintCMSStatistics = 0 {product} bool PrintClassHistogram = false {manageable} bool PrintClassHistogramAfterFullGC = false {manageable} bool PrintClassHistogramBeforeFullGC = false {manageable} bool PrintCodeCache = false {product} bool PrintCodeCacheOnCompilation = false {product} bool PrintCommandLineFlags = false {product} bool PrintCompilation = false {product} bool PrintConcurrentLocks = false {manageable} intx PrintFLSCensus = 0 {product} intx PrintFLSStatistics = 0 {product} bool PrintFlagsFinal := true {product} bool PrintFlagsInitial = false {product} bool PrintGC = false {manageable} bool PrintGCApplicationConcurrentTime = false {product} bool PrintGCApplicationStoppedTime = false {product} bool PrintGCCause = true {product} bool PrintGCDateStamps = false {manageable} bool PrintGCDetails = false {manageable} bool PrintGCID = false {manageable} bool PrintGCTaskTimeStamps = false {product} bool PrintGCTimeStamps = false {manageable} bool PrintHeapAtGC = false {product rw} bool PrintHeapAtGCExtended = false {product rw} bool PrintHeapAtSIGBREAK = true {product} bool PrintJNIGCStalls = false {product} bool PrintJNIResolving = false {product} bool PrintOldPLAB = false {product} bool PrintOopAddress = false {product} bool PrintPLAB = false {product} bool PrintParallelOldGCPhaseTimes = false {product} bool PrintPromotionFailure = false {product} bool PrintReferenceGC = false {product} bool PrintSafepointStatistics = false {product} intx PrintSafepointStatisticsCount = 300 {product} intx PrintSafepointStatisticsTimeout = -1 {product} bool PrintSharedArchiveAndExit = false {product} bool PrintSharedDictionary = false {product} bool PrintSharedSpaces = false {product} bool PrintStringDeduplicationStatistics = false {product} bool PrintStringTableStatistics = false {product} bool PrintTLAB = false {product} bool PrintTenuringDistribution = false {product} bool PrintTieredEvents = false {product} bool PrintVMOptions = false {product} bool PrintVMQWaitTime = false {product} bool PrintWarnings = true {product} uintx ProcessDistributionStride = 4 {product} bool ProfileInterpreter = true {pd product} bool ProfileIntervals = false {product} intx ProfileIntervalsTicks = 100 {product} intx ProfileMaturityPercentage = 20 {product} bool ProfileVM = false {product} bool ProfilerPrintByteCodeStatistics = false {product} bool ProfilerRecordPC = false {product} uintx PromotedPadding = 3 {product} uintx QueuedAllocationWarningCount = 0 {product} uintx RTMRetryCount = 5 {ARCH product} bool RangeCheckElimination = true {product} intx ReadPrefetchInstr = 0 {ARCH product} bool ReassociateInvariants = true {C2 product} bool ReduceBulkZeroing = true {C2 product} bool ReduceFieldZeroing = true {C2 product} bool ReduceInitialCardMarks = true {C2 product} bool ReduceSignalUsage = false {product} intx RefDiscoveryPolicy = 0 {product} bool ReflectionWrapResolutionErrors = true {product} bool RegisterFinalizersAtInit = true {product} bool RelaxAccessControlCheck = false {product} ccstr ReplayDataFile = {product} bool RequireSharedSpaces = false {product} uintx ReservedCodeCacheSize = 251658240 {pd product} bool ResizeOldPLAB = true {product} bool ResizePLAB = true {product} bool ResizeTLAB = true {pd product} bool RestoreMXCSROnJNICalls = false {product} bool RestrictContended = true {product} bool RewriteBytecodes = true {pd product} bool RewriteFrequentPairs = true {pd product} intx SafepointPollOffset = 256 {C1 pd product} intx SafepointSpinBeforeYield = 2000 {product} bool SafepointTimeout = false {product} intx SafepointTimeoutDelay = 10000 {product} bool ScavengeBeforeFullGC = true {product} intx SelfDestructTimer = 0 {product} uintx SharedBaseAddress = 34359738368 {product} ccstr SharedClassListFile = {product} uintx SharedMiscCodeSize = 122880 {product} uintx SharedMiscDataSize = 4194304 {product} uintx SharedReadOnlySize = 16777216 {product} uintx SharedReadWriteSize = 16777216 {product} bool ShowMessageBoxOnError = false {product} intx SoftRefLRUPolicyMSPerMB = 1000 {product} bool SpecialEncodeISOArray = true {C2 product} bool SplitIfBlocks = true {C2 product} intx StackRedPages = 1 {pd product} intx StackShadowPages = 20 {pd product} bool StackTraceInThrowable = true {product} intx StackYellowPages = 2 {pd product} bool StartAttachListener = false {product} intx StarvationMonitorInterval = 200 {product} bool StressLdcRewrite = false {product} uintx StringDeduplicationAgeThreshold = 3 {product} uintx StringTableSize = 60013 {product} bool SuppressFatalErrorMessage = false {product} uintx SurvivorPadding = 3 {product} uintx SurvivorRatio = 8 {product} intx SuspendRetryCount = 50 {product} intx SuspendRetryDelay = 5 {product} intx SyncFlags = 0 {product} ccstr SyncKnobs = {product} intx SyncVerbose = 0 {product} uintx TLABAllocationWeight = 35 {product} uintx TLABRefillWasteFraction = 64 {product} uintx TLABSize = 0 {product} bool TLABStats = true {product} uintx TLABWasteIncrement = 4 {product} uintx TLABWasteTargetPercent = 1 {product} uintx TargetPLABWastePct = 10 {product} uintx TargetSurvivorRatio = 50 {product} uintx TenuredGenerationSizeIncrement = 20 {product} uintx TenuredGenerationSizeSupplement = 80 {product} uintx TenuredGenerationSizeSupplementDecay = 2 {product} intx ThreadPriorityPolicy = 0 {product} bool ThreadPriorityVerbose = false {product} uintx ThreadSafetyMargin = 52428800 {product} intx ThreadStackSize = 1024 {pd product} uintx ThresholdTolerance = 10 {product} intx Tier0BackedgeNotifyFreqLog = 10 {product} intx Tier0InvokeNotifyFreqLog = 7 {product} intx Tier0ProfilingStartPercentage = 200 {product} intx Tier23InlineeNotifyFreqLog = 20 {product} intx Tier2BackEdgeThreshold = 0 {product} intx Tier2BackedgeNotifyFreqLog = 14 {product} intx Tier2CompileThreshold = 0 {product} intx Tier2InvokeNotifyFreqLog = 11 {product} intx Tier3BackEdgeThreshold = 60000 {product} intx Tier3BackedgeNotifyFreqLog = 13 {product} intx Tier3CompileThreshold = 2000 {product} intx Tier3DelayOff = 2 {product} intx Tier3DelayOn = 5 {product} intx Tier3InvocationThreshold = 200 {product} intx Tier3InvokeNotifyFreqLog = 10 {product} intx Tier3LoadFeedback = 5 {product} intx Tier3MinInvocationThreshold = 100 {product} intx Tier4BackEdgeThreshold = 40000 {product} intx Tier4CompileThreshold = 15000 {product} intx Tier4InvocationThreshold = 5000 {product} intx Tier4LoadFeedback = 3 {product} intx Tier4MinInvocationThreshold = 600 {product} bool TieredCompilation = true {pd product} intx TieredCompileTaskTimeout = 50 {product} intx TieredRateUpdateMaxTime = 25 {product} intx TieredRateUpdateMinTime = 1 {product} intx TieredStopAtLevel = 4 {product} bool TimeLinearScan = false {C1 product} bool TraceBiasedLocking = false {product} bool TraceClassLoading = false {product rw} bool TraceClassLoadingPreorder = false {product} bool TraceClassPaths = false {product} bool TraceClassResolution = false {product} bool TraceClassUnloading = false {product rw} bool TraceDynamicGCThreads = false {product} bool TraceGen0Time = false {product} bool TraceGen1Time = false {product} ccstr TraceJVMTI = {product} bool TraceLoaderConstraints = false {product rw} bool TraceMetadataHumongousAllocation = false {product} bool TraceMonitorInflation = false {product} bool TraceParallelOldGCTasks = false {product} intx TraceRedefineClasses = 0 {product} bool TraceSafepointCleanupTime = false {product} bool TraceSharedLookupCache = false {product} bool TraceSuspendWaitFailures = false {product} intx TrackedInitializationLimit = 50 {C2 product} bool TransmitErrorReport = false {product} bool TrapBasedNullChecks = false {pd product} bool TrapBasedRangeChecks = false {C2 pd product} intx TypeProfileArgsLimit = 2 {product} uintx TypeProfileLevel = 111 {pd product} intx TypeProfileMajorReceiverPercent = 90 {C2 product} intx TypeProfileParmsLimit = 2 {product} intx TypeProfileWidth = 2 {product} intx UnguardOnExecutionViolation = 0 {product} bool UnlinkSymbolsALot = false {product} bool Use486InstrsOnly = false {ARCH product} bool UseAES = true {product} bool UseAESIntrinsics = true {product} intx UseAVX = 2 {ARCH product} bool UseAdaptiveGCBoundary = false {product} bool UseAdaptiveGenerationSizePolicyAtMajorCollection = true {product} bool UseAdaptiveGenerationSizePolicyAtMinorCollection = true {product} bool UseAdaptiveNUMAChunkSizing = true {product} bool UseAdaptiveSizeDecayMajorGCCost = true {product} bool UseAdaptiveSizePolicy = true {product} bool UseAdaptiveSizePolicyFootprintGoal = true {product} bool UseAdaptiveSizePolicyWithSystemGC = false {product} bool UseAddressNop = true {ARCH product} bool UseAltSigs = false {product} bool UseAutoGCSelectPolicy = false {product} bool UseBMI1Instructions = true {ARCH product} bool UseBMI2Instructions = true {ARCH product} bool UseBiasedLocking = true {product} bool UseBimorphicInlining = true {C2 product} bool UseBoundThreads = true {product} bool UseCLMUL = true {ARCH product} bool UseCMSBestFit = true {product} bool UseCMSCollectionPassing = true {product} bool UseCMSCompactAtFullCollection = true {product} bool UseCMSInitiatingOccupancyOnly = false {product} bool UseCRC32Intrinsics = true {product} bool UseCodeCacheFlushing = true {product} bool UseCompiler = true {product} bool UseCompilerSafepoints = true {product} bool UseCompressedClassPointers := true {lp64_product} bool UseCompressedOops := true {lp64_product} bool UseConcMarkSweepGC = false {product} bool UseCondCardMark = false {C2 product} bool UseContainerSupport = true {product} bool UseCountLeadingZerosInstruction = true {ARCH product} bool UseCountTrailingZerosInstruction = true {ARCH product} bool UseCountedLoopSafepoints = false {C2 product} bool UseCounterDecay = true {product} bool UseDivMod = true {C2 product} bool UseDynamicNumberOfGCThreads = false {product} bool UseFPUForSpilling = true {C2 product} bool UseFastAccessorMethods = false {product} bool UseFastEmptyMethods = false {product} bool UseFastJNIAccessors = true {product} bool UseFastStosb = true {ARCH product} bool UseG1GC = false {product} bool UseGCLogFileRotation = false {product} bool UseGCOverheadLimit = true {product} bool UseGCTaskAffinity = false {product} bool UseHeavyMonitors = false {product} bool UseHugeTLBFS = false {product} bool UseInlineCaches = true {product} bool UseInterpreter = true {product} bool UseJumpTables = true {C2 product} bool UseLWPSynchronization = true {product} bool UseLargePages = false {pd product} bool UseLargePagesInMetaspace = false {product} bool UseLargePagesIndividualAllocation = false {pd product} bool UseLinuxPosixThreadCPUClocks = true {product} bool UseLockedTracing = false {product} bool UseLoopCounter = true {product} bool UseLoopInvariantCodeMotion = true {C1 product} bool UseLoopPredicate = true {C2 product} bool UseMathExactIntrinsics = true {C2 product} bool UseMaximumCompactionOnSystemGC = true {product} bool UseMembar = false {pd product} bool UseMontgomeryMultiplyIntrinsic = true {C2 product} bool UseMontgomerySquareIntrinsic = true {C2 product} bool UseMulAddIntrinsic = true {C2 product} bool UseMultiplyToLenIntrinsic = true {C2 product} bool UseNUMA = false {product} bool UseNUMAInterleaving = false {product} bool UseNewLongLShift = false {ARCH product} bool UseOSErrorReporting = false {pd product} bool UseOldInlining = true {C2 product} bool UseOnStackReplacement = true {pd product} bool UseOnlyInlinedBimorphic = true {C2 product} bool UseOprofile = false {product} bool UseOptoBiasInlining = true {C2 product} bool UsePSAdaptiveSurvivorSizePolicy = true {product} bool UseParNewGC = false {product} bool UseParallelGC := true {product} bool UseParallelOldGC = true {product} bool UsePerfData = true {product} bool UsePopCountInstruction = true {product} bool UseRDPCForConstantTableBase = false {C2 product} bool UseRTMDeopt = false {ARCH product} bool UseRTMLocking = false {ARCH product} bool UseSHA = false {product} bool UseSHA1Intrinsics = false {product} bool UseSHA256Intrinsics = false {product} bool UseSHA512Intrinsics = false {product} bool UseSHM = false {product} intx UseSSE = 4 {product} bool UseSSE42Intrinsics = true {product} bool UseSerialGC = false {product} bool UseSharedSpaces = false {product} bool UseSignalChaining = true {product} bool UseSquareToLenIntrinsic = true {C2 product} bool UseStoreImmI16 = false {ARCH product} bool UseStringDeduplication = false {product} bool UseSuperWord = true {C2 product} bool UseTLAB = true {pd product} bool UseThreadPriorities = true {pd product} bool UseTransparentHugePages = false {product} bool UseTypeProfile = true {product} bool UseTypeSpeculation = true {C2 product} bool UseUnalignedLoadStores = false {ARCH product} bool UseVMInterruptibleIO = false {product} bool UseXMMForArrayCopy = true {product} bool UseXmmI2D = false {ARCH product} bool UseXmmI2F = false {ARCH product} bool UseXmmLoadAndClearUpper = true {ARCH product} bool UseXmmRegToRegMoveAll = true {ARCH product} bool VMThreadHintNoPreempt = false {product} intx VMThreadPriority = -1 {product} intx VMThreadStackSize = 1024 {pd product} intx ValueMapInitialSize = 11 {C1 product} intx ValueMapMaxLoopSize = 8 {C1 product} intx ValueSearchLimit = 1000 {C2 product} bool VerifyMergedCPBytecodes = true {product} bool VerifySharedSpaces = false {product} intx WorkAroundNPTLTimedWaitHang = 1 {product} uintx YoungGenerationSizeIncrement = 20 {product} uintx YoungGenerationSizeSupplement = 80 {product} uintx YoungGenerationSizeSupplementDecay = 8 {product} uintx YoungPLABSize = 4096 {product} bool ZeroTLAB = false {product} intx hashCode = 5 {product}

添加JVM参数选项

运行jar包

java -Xms50m -Xmx50m -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -jar demo.jar

通过Tomcati运行war包

Linux系统下可以在tomcat/bin/catalina.sh中添加类似如下配置：JAVA_OPTS="-Xms512M -Xmx1024M"Windows系统下在catalina.bat中添加类似如下配置：set "JAVA_OPTS=-Xms512M -Xmx1024M"

程序运行过程中

使用jinfo -flag <name>=<value> <pid> 设置非Boolean类型参数使用jinfo -flag [+|-]<name> <pid> 设置Boolean类型参数

常用的JVM参数选项打印设置的XX选项及值

-XX:+PrintCommandLineFlags

可以让在程序运行前打印出用户手动设置或者JVM自动设置的XX选项

-XX:+PrintFlagsInitial

表示打印出所有XX选项的默认值

-XX:+PrintFlagsFinal

表示打印出X选项在运行程序时生效的值

-XX:+PrintVMOptions

打印JVM的参数

堆、栈、方法区等内存大小设置栈

-Xss128k

设置每个线程的栈大小为128k；等价于-XX:ThreadStackSize=128k

堆内存

-Xms3550m

等价于-XX:nitialHeapSize，设置JVM初始堆内存为3550M

-Xmx3550m

等价于-XX:MaxHeapSize，设置JVM最大堆内存为3550M

-Xmn2g

设置年轻代大小为2G；官方推荐配置为整个堆大小的3/8

-XX:NewSize=1024m

设置年轻代初始值为1024M

-XX:MaxNewSize=1024m

设置年轻代最大值为1024M

-XX:SurvivorRatio=8

设置年轻代中Eden区与一个Survivor区的比值，默认为8

-XX:+UseAdaptiveSizePolicy

自动选择名区大小比例

-XX:NewRatio=4

设置老年代与年轻代（包括1个Eden和2个Survivor区）的比值

-XX:PretenureSizeThreadshold=1024

设置让大于此阈值的对象直接分配在老年代，单位为字节；只对Serial、ParNew收集器有效

-XX:MaxTenuringThreshold=15

默认值为15；新生代每次MinorGC后，还存活的对象年龄+1，当对象的年龄大于设置的这个值时就进入老年代

-XX:+PrintTenuringDistribution

让JVM在每次MinorGC后打印出当前使用的Survivort中对象的年龄分布

-XX:TargetSurvivorRatio

表示MinorGC结束后Survivor区域中占用空间的期望比例

方法区

永久代

-XX:PermSize=256m

设置永久代初始值为256M

-XX:MaxPermSize=256m

设置永久代最大值为256M

元空间

-XX:MetaspaceSize

初始空间大小

-XX:MaxMetaspaceSize

最大空间，默认没有限制

-XX:+UseCompressedOops

压缩对象指针

-XX:+UseCompressedClassPointers

压缩类指针

-XX:CompressedClassSpaceSize

设置Klass Metaspacef的大小，默认1G

直接内存

-XX:MaxDirectMemorySize

指定DirectMemory容量，若未指定，则默认与Java堆最大值一样

OutofMemory相关的选项

-XX:+HeapDumpOnOutOfMemoryError

表示在内存出现OOM的时候，把Heap转存(Dump)到文件以便后续分析

-XX:+HeapDumpBeforeFullGC

表示在出现FullGC之前，生成Heap转储文件

-XX:HeapDumpPath=<path>

指定heap转存文件的存储路径

-XX:OnOutOfMemoryError

指定一个可行性程序或者脚本的路径，当发生OOM的时候，去执行这个脚本

对OnOutOfMemoryError的运维处理：

以部署在linux系统/opt/Server目录下的Server.jar为例

1.在run.sh启动脚本中添加jvm参数：

-XX:OnOutOfMemoryError=/opt/Server/restart.sh

2.restart.sh脚本

linux环境：

#!/bin/bash

pid=$(ps -ef|grep Server.jar|awk '{if($8=="java") {print $2}}')

kill -9 $pid

cd /opt/Server/;sh run.sh

Windows环境：

echo off

wmic process where Name='java.exe' delete

cd D:\Server

start run.bat

垃圾收集器相关选项

7款经典收集器与垃圾分代之间的关系

垃圾收集器的组合关系

1. 两个收集器间有连线，表明它们可以搭配使用：

Serial/Serial Old、Serial/CMS、ParNew/Serial Old、ParNew/CMS、Parallel Scavenge/Serial Old、Parallel Scavenge/Parallel Old、G1；

2. 其中Serial Old作为CMS出现"Concurrent Mode Failure"失败的后备预案。

3. (红色虚线)由于维护和兼容性测试的成本，在JDK 8时将Serial+CMS、ParNew+Serial Old这两个组合声明为废弃（JEP 173），并在JDK 9中完全取消了这些组合的支持（JEP214），即：移除。

4. (绿色虚线)JDK 14中：弃用Parallel Scavenge和SerialOld GC组合（JEP 366）

5. (青色虚线)JDK 14中：删除CMS垃圾回收器（JEP 363）

查看默认垃圾收集器 -XX:+PrintCommandLineFlags：查看命令行相关参数（包含使用的垃圾收集器）使用命令行指令：jinfo –flag 相关垃圾回收器参数进程ID Serial回收器

Serial收集器作为HotSpot中Client模式下的默认新生代垃圾收集器。Serial Old是运行在Client模式下默认的老年代的垃圾回收器。

-XX:+UseSerialGC

指定年轻代和老年代都使用串行收集器。等价于新生代用Serial GC，且老年代用Serial Old GC。可以获得最高的单线程收集效率。

ParNew回收器 -XX:+UseParNewGC

手动指定使用ParNew收集器执行内存回收任务。它表示年轻代使用并行收集器，不影响老年代。

-XX:ParallelGCThreads=N

限制线程数量，默认开启和CPU数据相同的线程数。

Parallel回收器 -XX:+UseParallelGC 手动指定年轻代使用Parallel并行收集器执行内存回收任务。-XX:+UseParallelOldGC 手动指定老年代都是使用并行回收收集器。分别适用于新生代和老年代。默认jdk8是开启的。上面两个参数，默认开启一个，另一个也会被开启。（互相激活）-XX:ParallelGCThreads 设置年轻代并行收集器的线程数。一般地，最好与CPU数量相等，以避免过多的线程数影响垃圾收集性能。在默认情况下，当CPU 数量小于8个， ParallelGCThreads 的值等于CPU 数量。当CPU数量大于8个，ParallelGCThreads 的值等于3+[5*CPU_Count]/8] 。-XX:MaxGCPauseMillis 设置垃圾收集器最大停顿时间(即STW的时间)。单位是毫秒。为了尽可能地把停顿时间控制在MaxGCPauseMills以内，收集器在工作时会调整Java堆大小或者其他一些参数。对于用户来讲，停顿时间越短体验越好。但是在服务器端，我们注重高并发，整体的吞吐量。所以服务器端适合Parallel，进行控制。该参数使用需谨慎。-XX:GCTimeRatio 垃圾收集时间占总时间的比例（= 1 / (N + 1))。用于衡量吞吐量的大小。取值范围（0,100）。默认值99，也就是垃圾回收时间不超过1%。与前一个-XX:MaxGCPauseMillis参数有一定矛盾性。暂停时间越长，Radio参数就容易超过设定的比例。-XX:+UseAdaptiveSizePolicy 设置Parallel Scavenge收集器具有自适应调节策略在这种模式下，年轻代的大小、Eden和Survivor的比例、晋升老年代的对象年龄等参数会被自动调整，已达到在堆大小、吞吐量和停顿时间之间的平衡点。在手动调优比较困难的场合，可以直接使用这种自适应的方式，仅指定虚拟机的最大堆、目标的吞吐量（GCTimeRatio）和停顿时间（MaxGCPauseMills），让虚拟机自己完成调优工作。 CMS回收器 -XX:+UseConcMarkSweepGC 手动指定使用CMS 收集器执行内存回收任务。开启该参数后会自动将-XX:+UseParNewGC打开。即：ParNew(Young区用)+CMS(Old区用)+Serial Old的组合。-XX:CMSlnitiatingOccupanyFraction 设置堆内存使用率的阈值，一旦达到该阈值，便开始进行回收。 JDK5及以前版本的默认值为68,即当老年代的空间使用率达到68%时，会执行一次CMS 回收。JDK6及以上版本默认值为92% 如果内存增长缓慢，则可以设置一个稍大的值，大的阈值可以有效降低CMS的触发频率，减少老年代回收的次数可以较为明显地改善应用程序性能。反之，如果应用程序内存使用率增长很快，则应该降低这个阈值，以避免频繁触发老年代串行收集器。因此通过该选项便可以有效降低Full GC 的执行次数。-XX:+UseCMSCompactAtFullCollection 用于指定在执行完Full GC后对内存空间进行压缩整理，以此避免内存碎片的产生。不过由于内存压缩整理过程无法并发执行，所带来的问题就是停顿时间变得更长了。-XX:CMSFullGCsBeforeCompaction 设置在执行多少次Full GC后对内存空间进行压缩整理。-XX:ParallelCMSThreads 设置CMS的线程数量。 CMS 默认启动的线程数是（ParallelGCThreads+3)/4，ParallelGCThreads 是年轻代并行收集器的线程数。当CPU 资源比较紧张时，受到CMS收集器线程的影响，应用程序的性能在垃圾回收阶段可能会非常糟糕。

另外，CMS收集器还有如下常用参数：

-XX:ConcGCThreads：设置并发垃圾收集的线程数，默认该值是基于ParallelGCThreads计算出来的；-XX:+UseCMSInitiatingOccupancyOnly：是否动态可调，用这个参数可以使CMS一直按CMSInitiatingOccupancyFraction设定的值启动-XX:+CMSScavengeBeforeRemark：强制hotspot虚拟机在cms remark阶段之前做一次minor gc，用于提高remark阶段的速度；-XX:+CMSClassUnloadingEnable：如果有的话，启用回收Perm 区（JDK8之前）-XX:+CMSParallelInitialEnabled：用于开启CMS initial-mark阶段采用多线程的方式进行标记，用于提高标记速度，在Java8开始已经默认开启；-XX:+CMSParallelRemarkEnabled：用户开启CMS remark阶段采用多线程的方式进行重新标记，默认开启；-XX:+ExplicitGCInvokesConcurrent 、-XX:+ExplicitGCInvokesConcurrentAndUnloadsClasses这两个参数用户指定hotspot虚拟在执行System.gc()时使用CMS周期;-XX:+CMSPrecleaningEnabled：指定CMS是否需要进行Pre cleaning这个阶段

说明：

JDK9新特性：CMS被标记为Deprecate了(JEP291) 如果对JDK 9及以上版本的HotSpot虚拟机使用参数-XX：+UseConcMarkSweepGC来开启CMS收集器的话，用户会收到一个警告信息，提示CMS未来将会被废弃。JDK14新特性：删除CMS垃圾回收器(JEP363) 移除了CMS垃圾收集器，如果在JDK14中使用-XX:+UseConcMarkSweepGC的话，JVM不会报错，只是给出一个warning信息，但是不会exit。JVM会自动回退以默认GC方式启动JVM

OpenJDK 64-Bit Server VM warning: Ignoring option UseConcMarkSweepGC; support was removed in 14.0 and the VM will continue execution using the default collector.

G1回收器 -XX：+UseG1GC

手动指定使用G1收集器执行内存回收任务。

-XX:G1HeapRegionSize

设置每个Region的大小。值是2的幂，范围是1MB到32MB之间，目标是根据最小的Java堆大小划分出约2048个区域。默认是堆内存的1/2000。

-XX:MaxGCPauseMillis

设置期望达到的最大GC停顿时间指标(JVM会尽力实现，但不保证达到)。默认值是200ms

-XX:ParallelGCThread

设置STW时GC线程数的值。最多设置为8

-XX:ConcGCThreads

设置并发标记的线程数。将n设置为并行垃圾回收线程数(ParallelGCThreads)的1/4左右。

-XX:InitiatingHeapOccupancyPercent

设置触发并发GC周期的Java堆占用率阈值。超过此值，就触发GC。默认值是45。

-XX:G1NewSizePercent、-XX:G1MaxNewSizePercent

新生代占用整个堆内存的最小百分比（默认5%）、最大百分比（默认60%）

-XX:G1ReservePercent=10

保留内存区域，防止 to space（Survivor中的to区）溢出

Mixed GC调优参数：

注意：G1收集器主要涉及到Mixed GC，Mixed GC会回收young 区和部分old区。

G1关于Mixed GC调优常用参数：

-XX:InitiatingHeapOccupancyPercent：设置堆占用率的百分比（0到100）达到这个数值的时候触发global concurrent marking（全局并发标记），默认为45%。值为0表示间断进行全局并发标记。-XX:G1MixedGCLiveThresholdPercent：设置Old区的region被回收时候的对象占比，默认占用率为85%。只有Old区的region中存活的对象占用达到了这个百分比，才会在Mixed GC中被回收。-XX:G1HeapWastePercent：在global concurrent marking（全局并发标记）结束之后，可以知道所有的区有多少空间要被回收，在每次young GC之后和再次发生Mixed GC之前，会检查垃圾占比是否达到此参数，只有达到了，下次才会发生Mixed GC。-XX:G1MixedGCCountTarget：一次global concurrent marking（全局并发标记）之后，最多执行Mixed GC的次数，默认是8。-XX:G1OldCSetRegionThresholdPercent：设置Mixed GC收集周期中要收集的Old region数的上限。默认值是Java堆的10% 怎么选择垃圾回收器优先调整堆的大小让JVM自适应完成。如果内存小于100M，使用串行收集器如果是单核、单机程序，并且没有停顿时间的要求，串行收集器如果是多CPU、需要高吞吐量、允许停顿时间超过1秒，选择并行或者JVM自己选择如果是多CPU、追求低停顿时间，需快速响应（比如延迟不能超过1秒，如互联网应用），使用并发收集器。官方推荐G1，性能高。现在互联网的项目，基本都是使用G1。

特别说明：

1. 没有最好的收集器，更没有万能的收集；

2. 调优永远是针对特定场景、特定需求，不存在一劳永逸的收集器

GC日志相关选项常用参数

-verbose:gc

输出gc日志信息，默认输出到标准输出；可以独立使用

-XX:+PrintGC

等同于-verbose:gc表示打开简化的GC日志；可以独立使用

-XX:+PrintGCDetails

在发生垃圾回收时打印内存回收详细的日志，并在进程退出时输出当前内存各区域分配情况；可以独立使用

-XX:+PrintGCTimeStamps

输出GC发生时的时间戳；不可以独立使用，需要配合-XX+PrintGCDetails使用

-XX:+PrintGCDateStamps

输出GC发生时的时间戳（以日期的形式，如2013-05-04T21:53:59,234+0800）；不可以独立使用，需要配合-XX+PrintGCDetails使用

-XX:+PrintHeapAtGC

每一次GC前和GC后，都打印堆信息；可以独立使用

-Xloggc:<file>

把GC日志写入到一个文件中去，而不是打印到标准输出中

其他参数

-XX:+TraceClassLoading

监控类的加载

-XX:PrintGCApplicationStoppedTime

打印GC时线程的停顿时间

-XX:+PrintGCApplicationConcurrentTime

垃圾收集之前打印出应用未中断的执行时间

-XX:+PrintReferenceGC

记录回收了多少种不同引用类型的引用

-XX:+PrintTenuringDistribution

让JVM在每次MinorGC后打印出当前使用的Survivor中对象的年龄分布

-XX:+UseGCLogFileRotation

启用GC日志文件的自动转储

-XX:NumberOfGClogFiles=1

GC日志文件的循环数目

-XX:GCLogFileSize=1M

控制GC日志文件的大小

其他参数

-XX:+DisableExplicitGC

禁止hotspot执行System.gc()，默认禁用

-XX:ReservedCodeCacheSize=<n> [g|m|k]、-XX:InitialCodeCacheSize=<n >[g|m|k]

指定代码缓存的大小

-XX:+UseCodeCacheFlushing

使用该参数让jvm放弃一些被编译的代码，避免代码缓存被占满时VM切换到interpreted-only的情况

-XX:+DoEscapeAnalysis

开启逃逸分析

-XX:+UseBiasedLocking

开启偏向锁

-XX:+UseLargePages

开启使用大页面

-XX:+UseTLAB

使用TLAB,默认打开

-XX:+PrintTLAB

打印TLAB的使用情况

-XX:TLABSize

设置TLAB大小

按功能性质区分JVM参数选项

Java HotSpot VM中-XX:的可配置参数列表进行描述；

这些参数可以被松散的聚合成三类：

行为参数（Behavioral Options）：用于改变jvm的一些基础行为；性能调优（Performance Tuning）：用于jvm的性能调优；调试参数（Debugging Options）：一般用于打开跟踪、打印、输出等jvm参数，用于显示jvm更加详细的信息；

行为参数(功能开关)

-XX:-DisableExplicitGC 禁止调用System.gc()；但jvm的gc仍然有效

-XX:+MaxFDLimit 最大化文件描述符的数量限制

-XX:+ScavengeBeforeFullGC 新生代GC优先于Full GC执行

-XX:+UseGCOverheadLimit 在抛出OOM之前限制jvm耗费在GC上的时间比例

-XX:-UseConcMarkSweepGC 对老生代采用并发标记交换算法进行GC

-XX:-UseParallelGC 启用并行GC

-XX:-UseParallelOldGC 对Full GC启用并行，当-XX:-UseParallelGC启用时该项自动启用

-XX:-UseSerialGC 启用串行GC

-XX:+UseThreadPriorities 启用本地线程优先级

性能调优

-XX:LargePageSizeInBytes=4m 设置用于Java堆的大页面尺寸

-XX:MaxHeapFreeRatio=70 GC后java堆中空闲量占的最大比例

-XX:MaxNewSize=size 新生成对象能占用内存的最大值

-XX:MaxPermSize=64m 老生代对象能占用内存的最大值

-XX:MinHeapFreeRatio=40 GC后java堆中空闲量占的最小比例

-XX:NewRatio=2 新生代内存容量与老生代内存容量的比例

-XX:NewSize=2.125m 新生代对象生成时占用内存的默认值

-XX:ReservedCodeCacheSize=32m 保留代码占用的内存容量

-XX:ThreadStackSize=512 设置线程栈大小，若为0则使用系统默认值

-XX:+UseLargePages 使用大页面内存

调试参数

-XX:-CITime 打印消耗在JIT编译的时间

-XX:ErrorFile=./hs_err_pid<pid>.log 保存错误日志或者数据到文件中

-XX:-ExtendedDTraceProbes 开启solaris特有的dtrace探针

-XX:HeapDumpPath=./java_pid<pid>.hprof 指定导出堆信息时的路径或文件名

-XX:-HeapDumpOnOutOfMemoryError 当首次遭遇OOM时导出此时堆中相关信息

-XX:OnError="<cmd args>;<cmd args>" 出现致命ERROR之后运行自定义命令

-XX:OnOutOfMemoryError="<cmd args>;<cmd args>" 当首次遭遇OOM时执行自定义命令

-XX:-PrintClassHistogram 遇到Ctrl-Break后打印类实例的柱状信息，与jmap -histo功能相同

-XX:-PrintConcurrentLocks 遇到Ctrl-Break后打印并发锁的相关信息，与jstack -l功能相同

-XX:-PrintCommandLineFlags 打印在命令行中出现过的标记

-XX:-PrintCompilation 当一个方法被编译时打印相关信息

-XX:-PrintGC 每次GC时打印相关信息

-XX:-PrintGC Details 每次GC时打印详细信息

-XX:-PrintGCTimeStamps 打印每次GC的时间戳

-XX:-TraceClassLoading 跟踪类的加载信息

-XX:-TraceClassLoadingPreorder 跟踪被引用到的所有类的加载信息

-XX:-TraceClassResolution 跟踪常量池

-XX:-TraceClassUnloading 跟踪类的卸载信息

-XX:-TraceLoaderConstraints 跟踪类加载器约束的相关信息

通过Java代码获取JVM参数

Java提供了java.lang.management包用于监视和管理Java虚拟机和Java运行时中的其他组件，它允许本地和远程监控和管理运行的Java虚拟机。其中ManagementFactory这个类还是挺常用的。另外还有Runtime类也可以获取一些内存、CPU核数等相关的数据。

通过这些api可以监控我们的应用服务器的堆内存使用情况，设置一些阈值进行报警等处理。

/** * * 监控我们的应用服务器的堆内存使用情况，设置一些阈值进行报警等处理 */ public class MemoryMonitor { public static void main(String[] args) { MemoryMXBean memorymbean = ManagementFactory.getMemoryMXBean(); MemoryUsage usage = memorymbean.getHeapMemoryUsage(); System.out.println("INIT HEAP: " + usage.getInit() / 1024 / 1024 + "m"); System.out.println("MAX HEAP: " + usage.getMax() / 1024 / 1024 + "m"); System.out.println("USE HEAP: " + usage.getUsed() / 1024 / 1024 + "m"); System.out.println("\nFull Information:"); System.out.println("Heap Memory Usage: " + memorymbean.getHeapMemoryUsage()); System.out.println("Non-Heap Memory Usage: " + memorymbean.getNonHeapMemoryUsage()); System.out.println("=======================通过java来获取相关系统状态============================ "); System.out.println("当前堆内存大小totalMemory " + (int) Runtime.getRuntime().totalMemory() / 1024 / 1024 + "m");// 当前堆内存大小 System.out.println("空闲堆内存大小freeMemory " + (int) Runtime.getRuntime().freeMemory() / 1024 / 1024 + "m");// 空闲堆内存大小 System.out.println("最大可用总堆内存maxMemory " + Runtime.getRuntime().maxMemory() / 1024 / 1024 + "m");// 最大可用总堆内存大小 } }

输出如下：

INIT HEAP: 100m MAX HEAP: 96m USE HEAP: 3m Full Information: Heap Memory Usage: init = 104857600(102400K) used = 3162256(3088K) committed = 100663296(98304K) max = 100663296(98304K) Non-Heap Memory Usage: init = 2555904(2496K) used = 5207432(5085K) committed = 8060928(7872K) max = -1(-1K) =======================通过java来获取相关系统状态======================== 当前堆内存大小totalMemory 96m 空闲堆内存大小freeMemory 92m 最大可用总堆内存maxMemory 96m

public class HeapSpaceInitial { public static void main(String[] args) { //返回Java虚拟机中的堆内存总量 long initialMemory = Runtime.getRuntime().totalMemory() / 1024 / 1024; //返回Java虚拟机试图使用的最大堆内存量 long maxMemory = Runtime.getRuntime().maxMemory() / 1024 / 1024; System.out.println("-Xms : " + initialMemory + "M"); System.out.println("-Xmx : " + maxMemory + "M"); System.out.println("系统内存大小为：" + maxMemory * 4.0 / 1024 + "G"); System.out.println("系统内存大小为：" + initialMemory * 64.0 / 1024 + "G"); } }

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