Minor GC和Full GC触发条件_minorgc什么时候触发-优快云博客

本文链接：https://blog.youkuaiyun.com/shi2huang/article/details/80067608

本文详细探讨了Java JVM中Minor GC和Full GC的触发条件。 Minor GC在eden区满时触发，将存活对象复制到to space，若to space不足则使用担保机制进入老年代。如果对象重写finalize方法，可能会影响其存活状态。Full GC可能由于老年代空间不足、System.gc()调用、年轻代晋升平均大小超过老年代可用内存以及eden和from space大于to space且大于老年代内存等情况触发。文章通过多个测试代码和分析展示了不同情况下的GC行为。

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一、Minor GC触发条件
1、eden区满时，触发MinorGC。即申请一个对象时，发现eden区不够用，则触发一次MinorGC。

注：新生代分为三个区域，eden space, from space, to space。默认比例是8:1:1。在MinorGC时，会把存活的对象复制到to space区域，如果to space区域不够，则利用担保机制进入老年代区域。

对eden space, from space, to space的理解：每次分配eden space空间，如果不够，则小于 to space大小的对象复制到 to space，然后to space和from space换位置，所以我们看到的to space一直是空的。

测试代码：

	private static final int _1M = 1024 * 1024;

	private static void testMinorGC() {

		/* eden space为8M，from/to space各为1M */
		LargeObject largeOb1 = new LargeObject(_1M * 1 / 2, "largeOb1");
		LargeObject largeOb2 = new LargeObject(_1M * 1, "largeOb2");
		LargeObject largeOb3 = new LargeObject(_1M * 2, "largeOb3");
		largeOb3 = null;
		LargeObject largeOb4 = new LargeObject(_1M * 3, "largeOb4");
		LargeObject largeOb5 = new LargeObject(_1M * 2, "largeOb5");
	}

	public static void main(String[] agrs) {
		testMinorGC();
	}

	static class LargeObject {
		private byte[] data;
		private String name;

		public LargeObject(int size, String name) {
			data = new byte[size];
			this.name = name;
			System.out.println("Over Constructing LargeObject " + name + System.lineSeparator());
		}

		public String getName() {
			return name;
		}
	}

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。在eclipse下运行参考eclipse设置运行JVM参数。

打印结果及分析：

Over Constructing LargeObject largeOb1

Over Constructing LargeObject largeOb2

Over Constructing LargeObject largeOb3

Over Constructing LargeObject largeOb4

{Heap before GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 7640K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 93% used [0x00000000ff600000,0x00000000ffd760d8,0x00000000ffe00000)
  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 2777K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 299K, capacity 386K, committed 512K, reserved 1048576K


// 实例化largeOb1、largeOb2、largeOb3、largeOb4共使用了eden space 6656K ，从上面信息可以看到eden space还剩余573m，初始化LargeOb5时，需要2048K。
// 由于eden space不够给新的对象分配内存，所以触发一次MinorGC。
// 7640K->1000K(9216K，eden space+from space)，新生代GC结果从7640K变成了1000K，这时新生代只存在largeOb1对应的内存，因为largeOb1对应的内存为0.5M，小于to space区大小，进入from space？不太懂。
// largeOb2和largeOb4由于对应的大于to space区大小，担保进入到老年代。
// 7640K->5232K(29696K，eden space+from space+ParOldGen)，总的内存占用变化，回收了2408K空间。
// largeOb3之前的内存没有引用（largeOb3=null），所以被回收了。
[GC (Allocation Failure) [PSYoungGen: 7640K->1000K(9216K)] 7640K->5248K(29696K), 0.0030601 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 7640K->616K(9216K)] 7640K->5232K(29696K), 0.0031851 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 


// 回收之后的内存情况，可以看到from space存在largeOb1，ParOldGen存在largeOb2和largeOb4。
Heap after GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 616K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 60% used [0x00000000ffe00000,0x00000000ffe9a020,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 4616K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 22% used [0x00000000fe200000,0x00000000fe682030,0x00000000ff600000)
 Metaspace       used 2777K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 299K, capacity 386K, committed 512K, reserved 1048576K
}
Over Constructing LargeObject largeOb5

// 分配完largeOb5，可以看到largeOb5的内存被分配在了新生代的eden space。
Heap
 PSYoungGen      total 9216K, used 2906K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 27% used [0x00000000ff600000,0x00000000ff83ca88,0x00000000ffe00000)
  from space 1024K, 60% used [0x00000000ffe00000,0x00000000ffe9a020,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 4616K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 22% used [0x00000000fe200000,0x00000000fe682030,0x00000000ff600000)
 Metaspace       used 2784K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 300K, capacity 386K, committed 512K, reserved 1048576K

2、Full GC之前调用，仅适用于 Parallel Scavenge（-XX:+UseParallelGC），虚拟机运行在Server模式下的默认收集器组合。

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。

测试代码：

	private static final int _1M = 1024 * 1024;

	private static void testMinorGCOverriddeFinalize() {


		/* Eden区为8M，from/to space各为1M */
		LargeObjectOverrideFinalize largeOb1 = new LargeObjectOverrideFinalize(_1M * 1 / 2, "largeOb1");
		LargeObjectOverrideFinalize largeOb2 = new LargeObjectOverrideFinalize(_1M * 1, "largeOb2");
		LargeObjectOverrideFinalize largeOb3 = new LargeObjectOverrideFinalize(_1M * 2, "largeOb3");
		largeOb1 = null;
		largeOb3 = null;
		LargeObjectOverrideFinalize largeOb4 = new LargeObjectOverrideFinalize(_1M * 3, "largeOb4");
		LargeObjectOverrideFinalize largeOb5 = new LargeObjectOverrideFinalize(_1M * 2, "largeOb5");


		/* 保证调用finalize完毕 */
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}


		System.gc();
	}

	public static void main(String[] agrs) {
		testMinorGCOverriddeFinalize();
	}

	static class LargeObjectOverrideFinalize {
		private byte[] data;
		private String name;

		public LargeObjectOverrideFinalize(int size, String name) {
			data = new byte[size];
			this.name = name;
			System.out.println("Over Constructing LargeObjectOverrideFinalize " + name + System.lineSeparator());
		}

		public String getName() {
			return name;
		}

		@Override
		public void finalize() {
			System.out.println(
					"F