- 引用计数法
在对象头处维护一个counter，每增加一次对该对象的引用计数器自加，如果对该对象的引用失联，则计数器自减。
当counter为0时，表明该对象已经被废弃，不处于存活状态。这种方式一方面无法区分软、虛、弱、强引用类别。
另一方面，会造成死锁，假设两个对象相互引用始终无法释放counter，永远不能GC。
-
可达性分析算法 （市面上比较常用）
- 虚拟机栈中引用的对象
- 方法区的类属性所引用的对象
- 方法区中常量所引用的对象
- 本地方法栈中引用的对象
-
验证jvm使用的哪种标记法
-verbose:gc -XX:+PrintGCDetails -XX:+PrintGCDateStamps
-verbose:gc == -XX:+PrintGC 打印gc
-XX:+PrintGCDetails 打印gc详情
-XX:+PrintGCDateStamps 打印gc时间
public class Biaoji {
private Object object;
private byte[] a = new byte[1024 * 1024 * 2];
public static void main(String[] args) {
Biaoji biaoji1 = new Biaoji();
Biaoji biaoji2 = new Biaoji();
biaoji1.object = biaoji2;
biaoji2.object = biaoji1;
biaoji1 = null;
biaoji2 = null;
System.gc();
}
}
[GC (System.gc()) [PSYoungGen: 8102K->688K(38400K)] 8102K->696K(125952K), 0.0017369 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
[Full GC (System.gc()) [PSYoungGen: 688K->0K(38400K)] [ParOldGen: 8K->606K(87552K)] 696K->606K(125952K), [Metaspace: 3343K->3343K(1056768K)], 0.0067050 secs] [Times: user=0.01 sys=0.00, real=0.00 secs]
Heap
PSYoungGen total 38400K, used 998K [0x0000000795580000, 0x0000000798000000, 0x00000007c0000000)
eden space 33280K, 3% used [0x0000000795580000,0x0000000795679b70,0x0000000797600000)
from space 5120K, 0% used [0x0000000797600000,0x0000000797600000,0x0000000797b00000)
to space 5120K, 0% used [0x0000000797b00000,0x0000000797b00000,0x0000000798000000)
ParOldGen total 87552K, used 606K [0x0000000740000000, 0x0000000745580000, 0x0000000795580000)
object space 87552K, 0% used [0x0000000740000000,0x0000000740097990,0x0000000745580000)
Metaspace used 3353K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 370K, capacity 388K, committed 512K, reserved 1048576K
Process finished with exit code 0
-XX:+PrintHeapAtGC 打印gc前后堆信息
image.png
invocations=1 表示共发生一次gc 。full 0 表示full gc发生了0 次
一次gc后对象由eden空间转移到了sorvivor form空间 与 old space
{Heap before GC invocations=1 (full 0):
PSYoungGen total 38400K, used 8102K [0x0000000795580000, 0x0000000798000000, 0x00000007c0000000)
eden space 33280K, 24% used [0x0000000795580000,0x0000000795d69b18,0x0000000797600000)
from space 5120K, 0% used [0x0000000797b00000,0x0000000797b00000,0x0000000798000000)
to space 5120K, 0% used [0x0000000797600000,0x0000000797600000,0x0000000797b00000)
ParOldGen total 87552K, used 0K [0x0000000740000000, 0x0000000745580000, 0x0000000795580000)
object space 87552K, 0% used [0x0000000740000000,0x0000000740000000,0x0000000745580000)
Metaspace used 3345K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 369K, capacity 388K, committed 512K, reserved 1048576K
[GC (System.gc()) [PSYoungGen: 8102K->688K(38400K)] 8102K->696K(125952K), 0.0020032 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
Heap after GC invocations=1 (full 0):
PSYoungGen total 38400K, used 688K [0x0000000795580000, 0x0000000798000000, 0x00000007c0000000)
eden space 33280K, 0% used [0x0000000795580000,0x0000000795580000,0x0000000797600000)
from space 5120K, 13% used [0x0000000797600000,0x00000007976ac010,0x0000000797b00000)
to space 5120K, 0% used [0x0000000797b00000,0x0000000797b00000,0x0000000798000000)
ParOldGen total 87552K, used 8K [0x0000000740000000, 0x0000000745580000, 0x0000000795580000)
object space 87552K, 0% used [0x0000000740000000,0x0000000740002000,0x0000000745580000)
Metaspace used 3345K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 369K, capacity 388K, committed 512K, reserved 1048576K
}
{Heap before GC invocations=2 (full 1):
PSYoungGen total 38400K, used 688K [0x0000000795580000, 0x0000000798000000, 0x00000007c0000000)
eden space 33280K, 0% used [0x0000000795580000,0x0000000795580000,0x0000000797600000)
from space 5120K, 13% used [0x0000000797600000,0x00000007976ac010,0x0000000797b00000)
to space 5120K, 0% used [0x0000000797b00000,0x0000000797b00000,0x0000000798000000)
ParOldGen total 87552K, used 8K [0x0000000740000000, 0x0000000745580000, 0x0000000795580000)
object space 87552K, 0% used [0x0000000740000000,0x0000000740002000,0x0000000745580000)
Metaspace used 3345K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 369K, capacity 388K, committed 512K, reserved 1048576K
[Full GC (System.gc()) [PSYoungGen: 688K->0K(38400K)] [ParOldGen: 8K->608K(87552K)] 696K->608K(125952K), [Metaspace: 3345K->3345K(1056768K)], 0.0066034 secs] [Times: user=0.01 sys=0.00, real=0.00 secs]
Heap after GC invocations=2 (full 1):
PSYoungGen total 38400K, used 0K [0x0000000795580000, 0x0000000798000000, 0x00000007c0000000)
eden space 33280K, 0% used [0x0000000795580000,0x0000000795580000,0x0000000797600000)
from space 5120K, 0% used [0x0000000797600000,0x0000000797600000,0x0000000797b00000)
to space 5120K, 0% used [0x0000000797b00000,0x0000000797b00000,0x0000000798000000)
ParOldGen total 87552K, used 608K [0x0000000740000000, 0x0000000745580000, 0x0000000795580000)
object space 87552K, 0% used [0x0000000740000000,0x0000000740098170,0x0000000745580000)
Metaspace used 3345K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 369K, capacity 388K, committed 512K, reserved 1048576K
}
Heap
PSYoungGen total 38400K, used 998K [0x0000000795580000, 0x0000000798000000, 0x00000007c0000000)
eden space 33280K, 3% used [0x0000000795580000,0x0000000795679b20,0x0000000797600000)
from space 5120K, 0% used [0x0000000797600000,0x0000000797600000,0x0000000797b00000)
to space 5120K, 0% used [0x0000000797b00000,0x0000000797b00000,0x0000000798000000)
ParOldGen total 87552K, used 608K [0x0000000740000000, 0x0000000745580000, 0x0000000795580000)
object space 87552K, 0% used [0x0000000740000000,0x0000000740098170,0x0000000745580000)
Metaspace used 3361K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 371K, capacity 388K, committed 512K, reserved 1048576K
Process finished with exit code 0
PSYoungGen 表示用的是年轻代使用Parallel Scavenge收集器。
PSOldGen 表示用的是老年代使用的Parallel Old收集器。
老年代的名称均为：ParOldGen；但是：-XX:+UseParallelGC和-XX:+UseParallelOldGC使用的老年代收集器不同，如下：
Option 新生代 老年代
-XX:+UseParallelGC Parallel Scavenge Serial Old
-XX:+UseParallelOldGC Parallel Scavenge Parallel Old
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