Permanent Generation Removal Overview英文

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1、1Copyright©2012,Oracleand/oritsaffiliates.Allrightsreserved.PermanentGenerationRemovalOverviewInsertPictureHereColeenPhillimoreJonMasamitsuOracleCorporationHotspotJVMTeam2Copyright©2012,Oracleand/oritsaffiliates.Allrightsreserved.ThepermanentgenerationinOra

2、cle’sHotSpotJVMheldmetadatathatHotSpotusedtodescribeJavaobjects.ThepermanentgenerationwasgarbagecollectedalongwiththeJavaheapduringfullGC.ThepermanentgenerationhasbeenremovedinHotSpotforJDK8.Thissessionbrieflydescribesthepermanentgenerationandthemotivationf

3、oritsremoval.ItalsodiscussessideeffectsoftheremovalthatmayaffecttheexecutionofaJavaapplication.3Copyright©2013,Oracleand/oritsaffiliates.Allrightsreserved.Theprecedingisintendedtooutlineourgeneralproductdirection.Itisintendedforinformationpurposesonly,andma

4、ynotbeincorporatedintoanycontract.Itisnotacommitmenttodeliveranymaterial,code,orfunctionality,andshouldnotberelieduponinmakingpurchasingdecisions.Thedevelopment,release,andtimingofanyfeaturesorfunctionalitydescribedforOracle’sproductsremainsatthesolediscret

5、ionofOracle.4Copyright©2012,Oracleand/oritsaffiliates.Allrightsreserved.ProgramAgenda§WhatwasthePermanentGeneration§WhereisMetadataNow§CompressedClassPointers§NewTuningFlags§MemoryPoolMXBeans5Copyright©2013,Oracleand/oritsaffiliates.Allrightsreserved.Whatwa

6、sPermGen?§The“Permanent”Generation§RegionofJavaHeapforJVMClassMetadata§Hotspot’sinternalrepresentationofJavaClasses●Classhierarchyinformation,fields,names●Methodcompilationinformationandbytecodes●Vtables●Constantpoolandsymbolicresolution6Copyright©2013,Orac

7、leand/oritsaffiliates.Allrightsreserved.JavaMemoryLayoutwithPermGenEdenSurvivorSurvivorOldGenerationforolderobjectsPermanentGenerationforVMmetadata7Copyright©2013,Oracleand/oritsaffiliates.Allrightsreserved.PermGenSize●LimitedtoMaxPermSize–default~64M-85M●C

8、ontiguouswithJavaHeap●Identifyingyoungreferencesfromoldgenandpermgenwouldbemoreexpensiveandcomplicatedwithanon-contiguousheap–cardtable●OnceexhaustedthrowsOutOfMemoryError“PermGenspace”●Applicationcoul