-XX:+PrintHeapAtGC 每次一次GC后，都打印堆信息

-XX:+PrintHeapAtGC
每次一次GC后，都打印堆信息

{Heap before GC invocations=0 (full 0):

 def new generation   total 3072K, used 2752K [0x33c80000, 0x33fd0000, 0x33fd0000)

  eden space 2752K, 100% used [0x33c80000, 0x33f30000, 0x33f30000)

  from space 320K,   0% used [0x33f30000, 0x33f30000, 0x33f80000)

  to   space 320K,   0% used [0x33f80000, 0x33f80000, 0x33fd0000)

 tenured generation   total 6848K, used 0K [0x33fd0000, 0x34680000, 0x34680000)

   the space 6848K,   0% used [0x33fd0000, 0x33fd0000, 0x33fd0200, 0x34680000)

 compacting perm gen  total 12288K, used 143K [0x34680000, 0x35280000, 0x38680000)

   the space 12288K,   1% used [0x34680000, 0x346a3c58, 0x346a3e00, 0x35280000)

    ro space 10240K,  44% used [0x38680000, 0x38af73f0, 0x38af7400, 0x39080000)

    rw space 12288K,  52% used [0x39080000, 0x396cdd28, 0x396cde00, 0x39c80000)

[GC[DefNew: 2752K->320K(3072K), 0.0014296 secs] 2752K->377K(9920K), 0.0014604 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]

Heap after GC invocations=1 (full 0):

 def new generation   total 3072K, used 320K [0x33c80000, 0x33fd0000, 0x33fd0000)

  eden space 2752K,   0% used [0x33c80000, 0x33c80000, 0x33f30000)

  from space 320K, 100% used [0x33f80000, 0x33fd0000, 0x33fd0000)

  to   space 320K,   0% used [0x33f30000, 0x33f30000, 0x33f80000)

 tenured generation   total 6848K, used 57K [0x33fd0000, 0x34680000, 0x34680000)

   the space 6848K,   0% used [0x33fd0000, 0x33fde458, 0x33fde600, 0x34680000)

 compacting perm gen  total 12288K, used 143K [0x34680000, 0x35280000, 0x38680000)

   the space 12288K,   1% used [0x34680000, 0x346a3c58, 0x346a3e00, 0x35280000)

    ro space 10240K,  44% used [0x38680000, 0x38af73f0, 0x38af7400, 0x39080000)

    rw space 12288K,  52% used [0x39080000, 0x396cdd28, 0x396cde00, 0x39c80000)

}