jvm：java中的引用（强引用、软引用、虚引用、弱引用）

1、分类

强引用、软引用、弱引用、虚引用、终结器引用

强引用：只要能够通过GC Root的引用链找到就不会被垃圾回收，也就是说只有所有的GC Roots对象都不通过强引用引用该对象的时候，该对象才能被垃圾回收

软引用：没有被其他强引用所引用的时候，当内存空间不够时，软引用的对象可能会被垃圾回收，可以配合引用队列来释放软引用自身

弱引用：没有被其他强引用所引用的时候，弱引用的对象就会被垃圾回收，不管内存是否充足，可以配合引用队列来释放弱引用自身

虚引用：创建的时候会关联一个引用队列，例如：创建ByteBuffer的时候会创建一个名为Cleaner的虚引用对象，当ByteBuffer没有被强引用所引用就会被jvm垃圾回收，虚引用Cleaner就会进入引用队列，会有专门的线程扫描引用队列，被发现后会调用直接内存地址的方法将直接内存释放掉，保证直接内存不会导致内存泄漏

终结器引用：创建的时候会关联一个引用队列，当A4对象没有被强引用所引用时，A4被垃圾回收的时候，会将终结器引用放入到一个引用队列（被引用对象暂时还没有被垃圾回收），有专门的线程（优先级较低，可能会造成对象迟迟不被回收）扫描引用队列并调用finallize（）方法，第二次GC的时候才能回收掉被引用对象

2、强引用

（1）强引用的使用：

设置jvm的内存为20M

public class Test2 {
    public static final int _4MB=4*1024*1024;
    public static void main(String[] args) throws IOException {
        List<byte[]> list =new ArrayList<>();
        for(int i=0;i<5;i++){
            list.add(new byte[_4MB]);
        }
        System.in.read();
    }
}

运行上面的程序会出现内存溢出问题，也就是说有的内存即使不太重要依旧会被占用：

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at pers.zhb.test.Test2.main(Test2.java:12)

3、软引用

（1）软引用的场景：

输入jvm指令：

public class Test2 {
    private static final int _4MB=4*1024*1024;
    public static void main(String[] args) throws IOException {
        soft();
    }
    public static void soft(){
        List<SoftReference<byte[]>> list=new ArrayList<>();
        for(int i=0;i<5;i++){
            SoftReference<byte[]> reference=new SoftReference<>(new byte[_4MB]);
            System.out.println(reference.get());
            list.add(reference);
            System.out.println(list.size());
        }
        System.out.println("循环结束："+list.size());
        for(SoftReference<byte[]> reference:list){
            System.out.println(reference.get());
        }
    }
}

运行结果：

[B@1b6d3586
1
[B@4554617c
2
[B@74a14482
3
[GC (Allocation Failure) [PSYoungGen: 1744K->488K(6144K)] 14032K->12932K(19968K), 0.0427814 secs] [Times: user=0.00 sys=0.00, real=0.05 secs] 
[B@1540e19d
4
[GC (Allocation Failure) --[PSYoungGen: 4696K->4696K(6144K)] 17140K->17140K(19968K), 0.0030222 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (Ergonomics) [PSYoungGen: 4696K->4574K(6144K)] [ParOldGen: 12444K->12414K(13824K)] 17140K->16988K(19968K), [Metaspace: 3118K->3118K(1056768K)], 0.0095409 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
[GC (Allocation Failure) --[PSYoungGen: 4574K->4574K(6144K)] 16988K->16996K(19968K), 0.0010492 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (Allocation Failure) [PSYoungGen: 4574K->0K(6144K)] [ParOldGen: 12422K->587K(8704K)] 16996K->587K(14848K), [Metaspace: 3118K->3118K(1056768K)], 0.0076471 secs] [Times: user=0.03 sys=0.00, real=0.01 secs] 
[B@677327b6
5
循环结束：5
null
null
null
null
[B@677327b6
Heap
 PSYoungGen      total 6144K, used 4546K [0x00000000ff980000, 0x0000000100000000, 0x0000000100000000)
  eden space 5632K, 80% used [0x00000000ff980000,0x00000000ffdf0940,0x00000000fff00000)
  from space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
  to   space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
 ParOldGen       total 8704K, used 587K [0x00000000fec00000, 0x00000000ff480000, 0x00000000ff980000)
  object space 8704K, 6% used [0x00000000fec00000,0x00000000fec92d20,0x00000000ff480000)
 Metaspace       used 3224K, capacity 4500K, committed 4864K, reserved 1056768K
  class space    used 349K, capacity 388K, committed 512K, reserved 1048576K

可以看出，在第四次和第五次的时候就触发了垃圾回收，在对集合进行遍历后只有第五个软引用对象存在，也就是说在内存不足的时候将前面的虚引用的对象进行了垃圾回收

（2）软引用与引用队列：

public class Test2 {
    private static final int _4MB=4*1024*1024;
    public static void main(String[] args) throws IOException {
        List<SoftReference<byte[]>> list=new ArrayList<>();
        //引用队列
        ReferenceQueue<byte[]> queue=new ReferenceQueue<>();
        for(int i=0;i<5;i++){
            //关联了引用队列，当软引用所关联的byte数组被回收时，软引用自己会加入到引用队列中
            SoftReference<byte[]> reference=new SoftReference<>(new byte[_4MB],queue);
            System.out.println(reference.get());
            list.add(reference);
            System.out.println(list.size());
        }
        //从队列获取无用的软引用对象并移除
        Reference<? extends byte[]> poll=queue.poll();
        while(poll!=null){
            list.remove(poll);
            poll=queue.poll();
        }
        System.out.println("循环结束");
        for(SoftReference<byte[]> reference:list){
            System.out.println(reference.get());
        }
    }
}

测试：

[B@1b6d3586
1
[B@4554617c
2
[B@74a14482
3
[GC (Allocation Failure) [PSYoungGen: 1866K->488K(6144K)] 14154K->12964K(19968K), 0.0010895 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[B@1540e19d
4
[GC (Allocation Failure) --[PSYoungGen: 4696K->4696K(6144K)] 17172K->17180K(19968K), 0.0011250 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (Ergonomics) [PSYoungGen: 4696K->4564K(6144K)] [ParOldGen: 12484K->12458K(13824K)] 17180K->17022K(19968K), [Metaspace: 3228K->3228K(1056768K)], 0.0055610 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
[GC (Allocation Failure) --[PSYoungGen: 4564K->4564K(6144K)] 17022K->17030K(19968K), 0.0009186 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (Allocation Failure) [PSYoungGen: 4564K->0K(6144K)] [ParOldGen: 12466K->620K(8704K)] 17030K->620K(14848K), [Metaspace: 3228K->3228K(1056768K)], 0.0065061 secs] [Times: user=0.00 sys=0.01, real=0.01 secs] 
[B@677327b6
5
循环结束
[B@677327b6
Heap
 PSYoungGen      total 6144K, used 4546K [0x00000000ff980000, 0x0000000100000000, 0x0000000100000000)
  eden space 5632K, 80% used [0x00000000ff980000,0x00000000ffdf0918,0x00000000fff00000)
  from space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
  to   space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
 ParOldGen       total 8704K, used 620K [0x00000000fec00000, 0x00000000ff480000, 0x00000000ff980000)
  object space 8704K, 7% used [0x00000000fec00000,0x00000000fec9b160,0x00000000ff480000)
 Metaspace       used 3237K, capacity 4500K, committed 4864K, reserved 1056768K
  class space    used 351K, capacity 388K, committed 512K, reserved 1048576K

Process finished with exit code 0

4、弱引用

（1）弱引用举例

public class Test2 {
    private static final int _4MB = 4 * 1024 * 1024;

    public static void main(String[] args) throws IOException {
        List<WeakReference<byte[]>> list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            WeakReference<byte[]> reference = new WeakReference<>(new byte[_4MB]);
            list.add(reference);
            for (WeakReference<byte[]> w : list) {
                System.out.println(w.get());
            }
            System.out.println();
        }
        System.out.println("循环结束：" + list.size());
    }
}

（2）测试：

[B@1b6d3586

[B@1b6d3586
[B@4554617c

[B@1b6d3586
[B@4554617c
[B@74a14482

[GC (Allocation Failure) [PSYoungGen: 1865K->488K(6144K)] 14153K->12992K(19968K), 0.0014680 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[B@1b6d3586
[B@4554617c
[B@74a14482
[B@1540e19d

[GC (Allocation Failure) [PSYoungGen: 4809K->496K(6144K)] 17313K->13044K(19968K), 0.0013554 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[B@1b6d3586
[B@4554617c
[B@74a14482
null
[B@677327b6

[GC (Allocation Failure) [PSYoungGen: 4702K->472K(6144K)] 17250K->13020K(19968K), 0.0017348 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[B@1b6d3586
[B@4554617c
[B@74a14482
null
null
[B@14ae5a5

[GC (Allocation Failure) [PSYoungGen: 4678K->488K(6144K)] 17226K->13036K(19968K), 0.0016909 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[B@1b6d3586
[B@4554617c
[B@74a14482
null
null
null
[B@7f31245a

[GC (Allocation Failure) [PSYoungGen: 4694K->488K(6144K)] 17242K->13044K(19968K), 0.0008993 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[B@1b6d3586
[B@4554617c
[B@74a14482
null
null
null
null
[B@6d6f6e28

[GC (Allocation Failure) [PSYoungGen: 4694K->472K(5120K)] 17250K->13028K(18944K), 0.0011246 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[B@1b6d3586
[B@4554617c
[B@74a14482
null
null
null
null
null
[B@135fbaa4

[GC (Allocation Failure) [PSYoungGen: 4657K->32K(5632K)] 17214K->13012K(19456K), 0.0012595 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (Ergonomics) [PSYoungGen: 32K->0K(5632K)] [ParOldGen: 12980K->638K(7680K)] 13012K->638K(13312K), [Metaspace: 3232K->3232K(1056768K)], 0.0084781 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
null
null
null
null
null
null
null
null
null
[B@45ee12a7

循环结束：10
Heap
 PSYoungGen      total 5632K, used 4371K [0x00000000ff980000, 0x0000000100000000, 0x0000000100000000)
  eden space 4608K, 94% used [0x00000000ff980000,0x00000000ffdc4e70,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 7680K, used 638K [0x00000000fec00000, 0x00000000ff380000, 0x00000000ff980000)
  object space 7680K, 8% used [0x00000000fec00000,0x00000000fec9fa38,0x00000000ff380000)
 Metaspace       used 3239K, capacity 4500K, committed 4864K, reserved 1056768K
  class space    used 351K, capacity 388K, committed 512K, reserved 1048576K

Process finished with exit code 0