jvm垃圾收集概述

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概述

 

代码如下

/**
 * -XX:+PrintGCDetails
 * -XX:+PrintGCDateStamps
 * -XX:+PrintGCTimeStamps
 */
public class ReferenceCountingGC {

    public Object instance = null;

    private static final int _1MB = 1024 * 1024;

    /**
     * 这个成员属性的唯一意义就是占点内存，以便在能在GC日志中看清楚是否有回收过
     */
    private byte[] bigSize = new byte[2 * _1MB];

    public static void testGC() {
        ReferenceCountingGC objA = new ReferenceCountingGC();
        ReferenceCountingGC objB = new ReferenceCountingGC();
        objA.instance = objB;
        objB.instance = objA;

        objA = null;
        objB = null;

        // 假设在这行发生GC，objA和objB是否能被回收？
        System.gc();
    }

    public static void main(String[] args) {
        testGC();
    }
}

gc日志如下

D:\Java\jdk1.8.0_172\bin\java.exe -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCTimeStamps "
Heap
 PSYoungGen      total 38400K, used 333K [0x00000000d5e00000, 0x00000000d8880000, 0x0000000100000000)
  eden space 33280K, 1% used [0x00000000d5e00000,0x00000000d5e534a8,0x00000000d7e80000)
  from space 5120K, 0% used [0x00000000d7e80000,0x00000000d7e80000,0x00000000d8380000)
  to   space 5120K, 0% used [0x00000000d8380000,0x00000000d8380000,0x00000000d8880000)
 ParOldGen       total 87552K, used 643K [0x0000000081a00000, 0x0000000086f80000, 0x00000000d5e00000)
  object space 87552K, 0% used [0x0000000081a00000,0x0000000081aa0e50,0x0000000086f80000)
 Metaspace       used 3444K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 376K, capacity 388K, committed 512K, reserved 1048576K

Process finished with exit code 0

因为我是jdk8，所以这部分和书上的不一样

代码如下

public class GC {

    public static GC SAVE_HOOK = null;

    public static void main(String[] args) throws InterruptedException {
        SAVE_HOOK = new GC();
        SAVE_HOOK = null;
        System.gc();
        Thread.sleep(500);
        if (null != SAVE_HOOK) { //此时对象应该处于(reachable, finalized)状态  
            System.out.println("Yes , I am still alive");
        } else {
            System.out.println("No , I am dead");
        }
        SAVE_HOOK = null;
        System.gc();
        Thread.sleep(500);
        if (null != SAVE_HOOK) {
            System.out.println("Yes , I am still alive");
        } else {
            System.out.println("No , I am dead");
        }
    }

    @Override
    protected void finalize() throws Throwable {
        super.finalize();
        System.out.println("execute method finalize()");
        SAVE_HOOK = this;
    }
}