输出Java的GC信息

-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

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]

-XX:+PrintGCTimeStamps 打印CG发生的时间戳,这个时间是相对于JVM启动时间的一个相对时间.

0.346: [GC0.346: [DefNew: 4416K->512K(4928K), 0.0047316 secs] 4416K->1189K(15872K), 0.0050057 secs] [Times: user=0.00 sys=0.02, real=0.02 secs] 
0.604: [GC0.604: [DefNew: 4928K->512K(4928K), 0.0047917 secs] 5605K->2042K(15872K), 0.0049892 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
0.862: [GC0.862: [DefNew: 4928K->512K(4928K), 0.0043302 secs] 6458K->2547K(15872K), 0.0045277 secs] [Times: user=0.02 sys=0.00, real=0.02 secs] 
1.097: [GC1.097: [DefNew: 4928K->511K(4928K), 0.0035988 secs] 6963K->2942K(15872K), 0.0037924 secs] [Times: user=0.00 sys=0.02, real=0.02 secs] 
1.332: [GC1.332: [DefNew: 4927K->511K(4928K), 0.0036309 secs] 7358K->3295K(15872K), 0.0038239 secs] [Times: user=0.01 sys=0.00, real=0.01 secs] 
1.501: [Full GC1.501: [Tenured: 2783K->2593K(10944K), 0.0277544 secs] 6493K->2593K(15872K), [Perm : 4095K->4095K(4096K)], 0.0279980 secs] [Times: user=0.03 sys=0.00, real=0.03 secs] 
1.529: [Full GC1.529: [Tenured: 2593K->2593K(10944K), 0.0228733 secs] 2593K->2593K(15936K), [Perm : 4095K->4095K(4096K)], 0.0229870 secs] [Times: user=0.02 sys=0.00, real=0.02 secs] 
1.555: [Full GC1.555: [Tenured: 2593K->999K(10944K), 0.0235103 secs] 2678K->999K(15936K), [Perm : 4095K->4095K(4096K)], 0.0236762 secs] [Times: user=0.02 sys=0.00, real=0.02 secs] 
1.579: [Full GC1.579: [Tenured: 999K->983K(10944K), 0.0265414 secs] 1045K->983K(15936K), [Perm : 4095K->4092K(4096K)], 0.0267126 secs] [Times: user=0.01 sys=0.00, real=0.03 secs] 
Heap
 def new generation   total 4992K, used 175K [0x26720000, 0x26c80000, 0x2bc70000)
  eden space 4480K,   3% used [0x26720000, 0x2674bd48, 0x26b80000)
  from space 512K,   0% used [0x26b80000, 0x26b80000, 0x26c00000)
  to   space 512K,   0% used [0x26c00000, 0x26c00000, 0x26c80000)
 tenured generation   total 10944K, used 983K [0x2bc70000, 0x2c720000, 0x36720000)
   the space 10944K,   8% used [0x2bc70000, 0x2bd65ce0, 0x2bd65e00, 0x2c720000)
 compacting perm gen  total 4096K, used 4094K [0x36720000, 0x36b20000, 0x36b20000)
   the space 4096K,  99% used [0x36720000, 0x36b1f948, 0x36b1fa00, 0x36b20000)
    ro space 10240K,  44% used [0x36b20000, 0x36f97e30, 0x36f98000, 0x37520000)
    rw space 12288K,  52% used [0x37520000, 0x37b6eb58, 0x37b6ec00, 0x38120000)

-XX:+PrintGCDateStamps 打印CG发生的时间戳,是一个阅读性好的,普通的日期时间.

java -jar -XX:+PrintGCDateStamps -XX:+PrintGCDetails -verbose:gc yed.jar

2011-05-11T17:49:43.959+0800: [GC [PSYoungGen: 15360K->2464K(17856K)] 15360K->8417K(58688K), 0.0260480 secs] [Times: user=0.05 sys=0.00, real=0.02 secs]

2011-05-11T17:49:44.612+0800: [GC [PSYoungGen: 17824K->2484K(33216K)] 23777K->10471K(74048K), 0.0156130 secs] [Times: user=0.03 sys=0.00, real=0.02 secs]

2011-05-11T17:49:44.724+0800: [GC [PSYoungGen: 33204K->2466K(33216K)] 41191K->10509K(74048K), 0.0088430 secs] [Times: user=0.02 sys=0.00, real=0.01 secs]

2011-05-11T17:49:44.771+0800: [GC [PSYoungGen: 33186K->2466K(63936K)] 41229K->10509K(104768K), 0.0069690 secs] [Times: user=0.02 sys=0.00, real=0.01 secs]

2011-05-11T17:49:44.911+0800: [GC [PSYoungGen: 63906K->2482K(63936K)] 71949K->10525K(104768K), 0.0071580 secs] [Times: user=0.02 sys=0.00, real=0.01 secs]

2011-05-11T17:49:47.282+0800: [GC [PSYoungGen: 63922K->8765K(130048K)] 71965K->16807K(170880K), 0.0409860 secs] [Times: user=0.07 sys=0.01, real=0.04 secs]

-XX:+PrintGCDetails的输出

Heap                          12288K+ 1536K            [低边界             当前边界       最高边界]

def new generation total 13824K, used 11223K [0x27e80000, 0x28d80000, 0x28d80000)                   （0x28d80000-0x27e80000)/1024/1024=15M

eden space 12288K, 91% used [0x27e80000, 0x28975f20, 0x28a80000)

from space 1536K, 0% used [0x28a80000, 0x28a80000, 0x28c00000)

to space 1536K, 0% used [0x28c00000, 0x28c00000, 0x28d80000)

tenured generation total 5120K, used 0K [0x28d80000, 0x29280000, 0x34680000)

the space 5120K, 0% used [0x28d80000, 0x28d80000, 0x28d80200, 0x29280000)

compacting perm gen total 12288K, used 142K [0x34680000, 0x35280000, 0x38680000) 

the space 12288K, 1% used [0x34680000, 0x346a3a90, 0x346a3c00, 0x35280000)

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

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

-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

目录可以是一个相对当前目录的相对地址，但是目录要存在，否则没有输出文件.