通过Java/JMX得到full GC次数

今天有个同事问如何能通过 JMX 获取到某个Java进程的full GC次数： 

引用

hi,问个问题，怎们在java中获取到full gc的次数呢？ 
我现在用jmx的那个得到了gc次数，不过不能细化出来full gc的次数 

 for (final GarbageCollectorMXBean garbageCollector
         : ManagementFactory.getGarbageCollectorMXBeans()) {
     gcCounts += garbageCollector.getCollectionCount();
 }

你比如我现在是这样拿次数的

我回答说因为full GC概念只有在分代式GC的上下文中才存在，而JVM并不强制要求GC使用分代式实现，所以JMX提供的标准 MXBean  API里不提供“full GC次数”这样的方法也正常。 
既然“full GC”本来就是非常平台相关的概念，那就hack一点，用平台相关的代码来解决问题好了。这些GC的MXBean都是有名字的，而主流的JVM的GC名字相对稳定，非要通过JMX得到full GC次数的话，用名字来判断一下就好了。 

举个例子来看看。通过JDK 6自带的 JConsole 工具来查看相关的MXBean的话，可以看到， 

GC的MXBean在这个位置： 
 

这个例子是用server模式启动JConsole的，使用的是ParallelScavenge GC，它的年老代对应的收集器在这里： 
 

该收集器的总收集次数在此，这也就是full GC的次数： 
 

于是只要知道我们用的JVM提供的GC MXBean的名字与分代的关系，就可以知道full GC的次数了。 
Java代码写起来冗长，这帖就不用Java来写例子了，反正API是一样的，意思能表达清楚就OK。 
用一个 Groovy 脚本简单演示一下适用于Oracle (Sun) HotSpot与Oracle (BEA) JRockit的GC统计程序： 

 import java.lang.management.ManagementFactory

 printGCStats = {
   def youngGenCollectorNames = [
     // Oracle (Sun) HotSpot
     // -XX:+UseSerialGC
     'Copy',
     // -XX:+UseParNewGC
     'ParNew',
     // -XX:+UseParallelGC
     'PS Scavenge',

     // Oracle (BEA) JRockit
     // -XgcPrio:pausetime
     'Garbage collection optimized for short pausetimes Young Collector',
     // -XgcPrio:throughput
     'Garbage collection optimized for throughput Young Collector',
     // -XgcPrio:deterministic
     'Garbage collection optimized for deterministic pausetimes Young Collector'
   ]

   def oldGenCollectorNames = [
     // Oracle (Sun) HotSpot
     // -XX:+UseSerialGC
     'MarkSweepCompact',
     // -XX:+UseParallelGC and (-XX:+UseParallelOldGC or -XX:+UseParallelOldGCCompacting)
     'PS MarkSweep',
     // -XX:+UseConcMarkSweepGC
     'ConcurrentMarkSweep',

     // Oracle (BEA) JRockit
     // -XgcPrio:pausetime
     'Garbage collection optimized for short pausetimes Old Collector',
     // -XgcPrio:throughput
     'Garbage collection optimized for throughput Old Collector',
     // -XgcPrio:deterministic
     'Garbage collection optimized for deterministic pausetimes Old Collector'
   ]

   R: {
     ManagementFactory.garbageCollectorMXBeans.each {
       def name  = it.name
       def count = it.collectionCount
       def gcType;
       switch (name) {
       case youngGenCollectorNames:
         gcType = 'Minor Collection'
         break
       case oldGenCollectorNames:
         gcType = 'Major Collection'
         break
       default:
         gcType = 'Unknown Collection Type'
         break
       }
       println "$count <- $gcType: $name"
     }
   }
 }

 printGCStats()

执行可以看到类似这样的输出： 

 5 <- Minor Collection: Copy
 0 <- Major Collection: MarkSweepCompact

↑这是用client模式的HotSpot执行得到的； 

 0 <- Minor Collection: Garbage collection optimized for throughput Young Collector
 0 <- Major Collection: Garbage collection optimized for throughput Old Collector

↑这是用JRockit R28在32位Windows上的默认模式得到的。 

通过上述方法，要包装起来方便以后使用的话也很简单，例如下面Groovy程序： 

 import java.lang.management.ManagementFactory

 class GCStats {
   static final List YoungGenCollectorNames = [
     // Oracle (Sun) HotSpot
     // -XX:+UseSerialGC
     'Copy',
     // -XX:+UseParNewGC
     'ParNew',
     // -XX:+UseParallelGC
     'PS Scavenge',

     // Oracle (BEA) JRockit
     // -XgcPrio:pausetime
     'Garbage collection optimized for short pausetimes Young Collector',
     // -XgcPrio:throughput
     'Garbage collection optimized for throughput Young Collector',
     // -XgcPrio:deterministic
     'Garbage collection optimized for deterministic pausetimes Young Collector'
   ]

   static final List OldGenCollectorNames = [
     // Oracle (Sun) HotSpot
     // -XX:+UseSerialGC
     'MarkSweepCompact',
     // -XX:+UseParallelGC and (-XX:+UseParallelOldGC or -XX:+UseParallelOldGCCompacting)
     'PS MarkSweep',
     // -XX:+UseConcMarkSweepGC
     'ConcurrentMarkSweep',

     // Oracle (BEA) JRockit
     // -XgcPrio:pausetime
     'Garbage collection optimized for short pausetimes Old Collector',
     // -XgcPrio:throughput
     'Garbage collection optimized for throughput Old Collector',
     // -XgcPrio:deterministic
     'Garbage collection optimized for deterministic pausetimes Old Collector'
   ]

   static int getYoungGCCount() {
     ManagementFactory.garbageCollectorMXBeans.inject(0) { youngGCCount, gc ->
       if (YoungGenCollectorNames.contains(gc.name))
         youngGCCount + gc.collectionCount
       else
         youngGCCount
     }
   }

   static int getFullGCCount() {
     ManagementFactory.garbageCollectorMXBeans.inject(0) { fullGCCount, gc ->
       if (OldGenCollectorNames.contains(gc.name))
         fullGCCount + gc.collectionCount
       else
         fullGCCount
     }
   }
 }

用的时候： 

 D:\>\sdk\groovy-1.7.2\bin\groovysh
 Groovy Shell (1.7.2, JVM: 1.6.0_20)
 Type 'help' or '\h' for help.
 --------------------------------------------------
 groovy:000> GCStats.fullGCCount
 ===> 0
 groovy:000> System.gc()
 ===> null
 groovy:000> GCStats.fullGCCount
 ===> 1
 groovy:000> System.gc()
 ===> null
 groovy:000> System.gc()
 ===> null
 groovy:000> GCStats.fullGCCount
 ===> 3
 groovy:000> GCStats.youngGCCount
 ===> 9
 groovy:000> GCStats.youngGCCount
 ===> 9
 groovy:000> GCStats.youngGCCount
 ===> 9
 groovy:000> System.gc()
 ===> null
 groovy:000> GCStats.youngGCCount
 ===> 9
 groovy:000> GCStats.fullGCCount
 ===> 4
 groovy:000> quit

这是在Sun JDK 6 update 20上跑的。顺带一提，如果这是跑在JRockit上的话，那full GC的次数就不会增加——因为JRockit里System.gc()默认是触发young GC的；请不要因为Sun HotSpot的默认行为而认为System.gc()总是会触发full GC的。 

Poonam Bajaj以前也写过一篇blog提到HotSpot VM里的GC MBean的名字的： 
Collector names for GarbageCollectorMXBean MXBean  

关于JMX的MXBean的使用，也可以参考下面两篇文档： 
Groovy and JMX  
Monitoring the JVM Heap with JRuby