jvm垃圾收集概述

迷南。 2022-05-14 14:12 164阅读 0赞

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# 概述 #

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# ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3VvdGFpbA_size_27_color_FFFFFF_t_70 2][] #

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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

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3VvdGFpbA_size_27_color_FFFFFF_t_70 7][]

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

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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;
        }
    }

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