关于垃圾回收的触发机制，参考MinorGC和FullGC触发条件，本篇就是从其扩展而来。
判断一个对象是否存活，除了GC Roots引用之外，还有一个条件就是对象是否重写了finalize方法，如果对象重写了该方法，则会交给FQueue队列去执行，如果执行该方法后被重新关联，则在下次回收时不会被回收，否则下次回收，该方法只执行一次。
注：如果在finalize方法里面被重新引用，则下次GC不会被回收。但是如果再次失去引用，则无论如何都会在下次GC时被回收，因为finalize方法只执行一次。
测试代码：
private static final int _1M = 1024 * 1024;
private static List<LargeObjectOverrideFinalizeEscapeCollect> escapeObjs = new ArrayList<LargeObjectOverrideFinalizeEscapeCollect>();
private static void testMinorGCOverriddeFinalizeEscapeCollect() {
/* Eden区为8M，from/to space各为1M */
LargeObjectOverrideFinalizeEscapeCollect largeOb1 = new LargeObjectOverrideFinalizeEscapeCollect(_1M * 1 / 2,
"largeOb1");
LargeObjectOverrideFinalizeEscapeCollect largeOb2 = new LargeObjectOverrideFinalizeEscapeCollect(_1M * 1,
"largeOb2");
LargeObjectOverrideFinalizeEscapeCollect largeOb3 = new LargeObjectOverrideFinalizeEscapeCollect(_1M * 2,
"largeOb3");
largeOb1 = null;
largeOb3 = null;
LargeObjectOverrideFinalizeEscapeCollect largeOb4 = new LargeObjectOverrideFinalizeEscapeCollect(_1M * 3,
"largeOb4");
LargeObjectOverrideFinalizeEscapeCollect largeOb5 = new LargeObjectOverrideFinalizeEscapeCollect(_1M * 2,
"largeOb5");
/* 保证调用finalize完毕 */
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.gc();
escapeObjs.clear();
System.out.println("=============clear escape objects.");
try {
Thread.sleep(10000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.gc();
}
public static void main(String[] agrs) {
testMinorGCOverriddeFinalizeEscapeCollect();
}
static class LargeObjectOverrideFinalizeEscapeCollect {
private byte[] data;
private String name;
public LargeObjectOverrideFinalizeEscapeCollect(int size, String name) {
data = new byte[size];
this.name = name;
System.out.println(
"Over Constructing LargeObjectOverrideFinalizeEscapeCollect " + name + System.lineSeparator());
}
public String getName() {
return name;
}
@Override
public void finalize() {
System.out.println("Finalize LargeObjectOverrideFinalizeEscapeCollect " + name + " , release "
+ (data.length / 1024) + "K.");
escapeObjs.add(this);
}
}
测试环境：JDK1.8.0_144，Java HotSpot(TM) 64-Bit Server VM，默认垃圾收集器为PS Scavenge+PS MarkSweep。
-Xms30m -Xmx30m -Xmn10m -XX:+PrintGCDetails -XX:+PrintHeapAtGC
堆总大小为30m，新生代为10m，打印GC信息，在GC前后打印内存信息。
当前垃圾处理器装置
打印结果及分析：
Over Constructing LargeObjectOverrideFinalizeEscapeCollect largeOb1
Over Constructing LargeObjectOverrideFinalizeEscapeCollect largeOb2
Over Constructing LargeObjectOverrideFinalizeEscapeCollect largeOb3
Over Constructing LargeObjectOverrideFinalizeEscapeCollect largeOb4
{Heap before GC invocations=1 (full 0):
PSYoungGen total 9216K, used 7804K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 95% used [0x00000000ff600000,0x00000000ffd9f030,0x00000000ffe00000)
from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
to space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
ParOldGen total 20480K, used 0K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
object space 20480K, 0% used [0x00000000fe200000,0x00000000fe200000,0x00000000ff600000)
Metaspace used 2867K, capacity 4490K, committed 4864K, reserved 1056768K
class space used 311K, capacity 386K, committed 512K, reserved 1048576K
// 由于重写了finalize，所以第一次GC时largeOb1和largeOb3没有被回收。
[GC (Allocation Failure) [PSYoungGen: 7804K->664K(9216K)] 7804K->7328K(29696K), 0.0044152 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
Heap after GC invocations=1 (full 0):
PSYoungGen total 9216K, used 664K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
from space 1024K, 64% used [0x00000000ffe00000,0x00000000ffea6030,0x00000000fff00000)
to space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
ParOldGen total 20480K, used 6664K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
object space 20480K, 32% used [0x00000000fe200000,0x00000000fe882040,0x00000000ff600000)
Metaspace used 2867K, capacity 4490K, committed 4864K, reserved 1056768K
class space used 311K, capacity 386K, committed 512K, reserved 1048576K
}
Finalize LargeObjectOverrideFinalizeEscapeCollect largeOb1 , release 512K.
Over Constructing LargeObjectOverrideFinalizeEscapeCollect largeOb5
Finalize LargeObjectOverrideFinalizeEscapeCollect largeOb3 , release 2048K.
{Heap before GC invocations=2 (full 0):
PSYoungGen total 9216K, used 3036K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 28% used [0x00000000ff600000,0x00000000ff851140,0x00000000ffe00000)
from space 1024K, 64% used [0x00000000ffe00000,0x00000000ffea6030,0x00000000fff00000)
to space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
ParOldGen total 20480K, used 6664K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
object space 20480K, 32% used [0x00000000fe200000,0x00000000fe882040,0x00000000ff600000)
Metaspace used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
class space used 311K, capacity 386K, committed 512K, reserved 1048576K
// 由于被重新关联，所以第二次GC时largeOb1和largeOb3也没有被回收
[GC (System.gc()) [PSYoungGen: 3036K->656K(9216K)] 9700K->9368K(29696K), 0.0036913 secs] [Times: user=0.06 sys=0.00, real=0.00 secs]
Heap after GC invocations=2 (full 0):
PSYoungGen total 9216K, used 656K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
from space 1024K, 64% used [0x00000000fff00000,0x00000000fffa4040,0x0000000100000000)
to space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
ParOldGen total 20480K, used 8712K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
object space 20480K, 42% used [0x00000000fe200000,0x00000000fea82050,0x00000000ff600000)
Metaspace used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
class space used 311K, capacity 386K, committed 512K, reserved 1048576K
}
{Heap before GC invocations=3 (full 1):
PSYoungGen total 9216K, used 656K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
from space 1024K, 64% used [0x00000000fff00000,0x00000000fffa4040,0x0000000100000000)
to space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
ParOldGen total 20480K, used 8712K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
object space 20480K, 42% used [0x00000000fe200000,0x00000000fea82050,0x00000000ff600000)
Metaspace used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
class space used 311K, capacity 386K, committed 512K, reserved 1048576K
// 由于被重新关联，所以第一次Full GC时largeOb1和largeOb3没有被回收。
[Full GC (System.gc()) [PSYoungGen: 656K->0K(9216K)] [ParOldGen: 8712K->9264K(20480K)] 9368K->9264K(29696K), [Metaspace: 2868K->2868K(1056768K)], 0.0148887 secs] [Times: user=0.06 sys=0.00, real=0.02 secs]
Heap after GC invocations=3 (full 1):
PSYoungGen total 9216K, used 0K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
to space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
ParOldGen total 20480K, used 9264K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
object space 20480K, 45% used [0x00000000fe200000,0x00000000feb0c128,0x00000000ff600000)
Metaspace used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
class space used 311K, capacity 386K, committed 512K, reserved 1048576K
}
=============clear escape objects.
{Heap before GC invocations=4 (full 1):
PSYoungGen total 9216K, used 160K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 1% used [0x00000000ff600000,0x00000000ff6281a8,0x00000000ffe00000)
from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
to space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
ParOldGen total 20480K, used 9264K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
object space 20480K, 45% used [0x00000000fe200000,0x00000000feb0c128,0x00000000ff600000)
Metaspace used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
class space used 311K, capacity 386K, committed 512K, reserved 1048576K
// 这次GC只有新生代的GC，老年代没有GC，但是又叫Full GC，比较疑惑？
[GC (System.gc()) [PSYoungGen: 160K->32K(9216K)] 9424K->9296K(29696K), 0.0005980 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap after GC invocations=4 (full 1):
PSYoungGen total 9216K, used 32K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
from space 1024K, 3% used [0x00000000ffe00000,0x00000000ffe08000,0x00000000fff00000)
to space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
ParOldGen total 20480K, used 9264K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
object space 20480K, 45% used [0x00000000fe200000,0x00000000feb0c128,0x00000000ff600000)
Metaspace used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
class space used 311K, capacity 386K, committed 512K, reserved 1048576K
}
{Heap before GC invocations=5 (full 2):
PSYoungGen total 9216K, used 32K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
from space 1024K, 3% used [0x00000000ffe00000,0x00000000ffe08000,0x00000000fff00000)
to space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
ParOldGen total 20480K, used 9264K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
object space 20480K, 45% used [0x00000000fe200000,0x00000000feb0c128,0x00000000ff600000)
Metaspace used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
class space used 311K, capacity 386K, committed 512K, reserved 1048576K
// 由于清理了引用，所以第二次Full GC时largeOb1和largeOb3被回收
[Full GC (System.gc()) [PSYoungGen: 32K->0K(9216K)] [ParOldGen: 9264K->6704K(20480K)] 9296K->6704K(29696K), [Metaspace: 2868K->2868K(1056768K)], 0.0095751 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
Heap after GC invocations=5 (full 2):
PSYoungGen total 9216K, used 0K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
from space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
to space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
ParOldGen total 20480K, used 6704K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
object space 20480K, 32% used [0x00000000fe200000,0x00000000fe88c148,0x00000000ff600000)
Metaspace used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
class space used 311K, capacity 386K, committed 512K, reserved 1048576K
}
Heap
PSYoungGen total 9216K, used 164K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 2% used [0x00000000ff600000,0x00000000ff6290d0,0x00000000ffe00000)
from space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
to space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
ParOldGen total 20480K, used 6704K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
object space 20480K, 32% used [0x00000000fe200000,0x00000000fe88c148,0x00000000ff600000)
Metaspace used 2874K, capacity 4490K, committed 4864K, reserved 1056768K
class space used 312K, capacity 386K, committed 512K, reserved 1048576K
有一个小问题：为什么full 1有两次，第二次full 1只清理了新生代空间？
如果不执行escapeObjs.clear();，则对象都不会被回收。