3.常用JVM配置参数配置

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• Trace跟踪参数
• 堆的分配参数
• 栈的分配参数

Trace跟踪参数

• -verbose:gc

• -XX:+printGC

• 可以打印GC的简要信息

  • [GC 4790K->374K(15872K), 0.0001606 secs]
  • [GC 4790K->374K(15872K), 0.0001474 secs]
  • [GC 4790K->374K(15872K), 0.0001563 secs]
  • [GC 4790K->374K(15872K), 0.0001682 secs]

• -XX:+PrintGCDetails

  • 打印GC详细信息

• -XX:+PrintGCTimeStamps

  • 打印CG发生的时间戳
    例

[GC[DefNew: 4416K->0K(4928K), 0.0001897 secs] 4790K->374K(15872K), 0.0002232 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 

• -Xloggc:log/gc.log

  • 指定GC log的位置，以文件输出
  • 帮助开发人员分析问题
    

• -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)
}

• -XX:+TraceClassLoading
  • 监控类的加载
    • [Loaded java.lang.Object from shared objects file]
    • [Loaded java.io.Serializable from shared objects file]
    • [Loaded java.lang.Comparable from shared objects file]
    • [Loaded java.lang.CharSequence from shared objects file]
    • [Loaded java.lang.String from shared objects file]
    • [Loaded java.lang.reflect.GenericDeclaration from shared objects file]
    • [Loaded java.lang.reflect.Type from shared objects file]
• -XX:+PrintClassHistogram
  • 按下Ctrl+Break后，打印类的信息：

 num     #instances         #bytes  class name
----------------------------------------------
   1:        890617      470266000  [B
   2:        890643       21375432  java.util.HashMap$Node
   3:        890608       14249728  java.lang.Long
   4:            13        8389712  [Ljava.util.HashMap$Node;
   5:          2062         371680  [C
   6:           463          41904  java.lang.Class

• 分别显示：序号、实例数量、总大小、类型

堆的分配参数