內(nèi)容：

    理解JVM中PSYoungGen、ParOldGen、MetaSpace

一、理解JVM中PSYoungGen、ParOldGen、MetaSpace

Heap
 PSYoungGen      total 2560K, used 2505K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)
  eden space 2048K, 98% used [0x00000000ffd00000,0x00000000ffef64d8,0x00000000fff00000)
  from space 512K, 97% used [0x00000000fff00000,0x00000000fff7c2f0,0x00000000fff80000)
  to   space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
 ParOldGen       total 7168K, used 1196K [0x00000000ff600000, 0x00000000ffd00000, 0x00000000ffd00000)
  object space 7168K, 16% used [0x00000000ff600000,0x00000000ff72b180,0x00000000ffd00000)
 Metaspace       used 3377K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 352K, capacity 388K, committed 512K, reserved 1048576K

    PSYoungGen:GC日志中的PSYoungGen（PS是指Parallel Scavenge）為Eden+FromSpace，而整個YoungGeneration為Eden+FromSpace+ToSpace。
    ParOldGen:ParOldGen表示gc回收前后老年代的內(nèi)存變化
    MetaSpace:JDK1.7中，存儲在永久代的部分數(shù)據(jù)就已經(jīng)轉(zhuǎn)移到了Java Heap或者是 Native Heap。但永久代仍存在于JDK1.7中，并沒完全移除，譬如符號引用(Symbols)轉(zhuǎn)移到了native heap；字面量(interned strings)轉(zhuǎn)移到了java heap；類的靜態(tài)變量(class statics)轉(zhuǎn)移到了java heap。