java中的引用类型之强软弱虚详解

public class M {

 /**
  * 当这个对象会被回收的时候，finalize会被调用
  *
  * @throws Throwable
  */
 @Override
 protected void finalize() throws Throwable {
  System.out.println("finalize");
 }
}

public class NormalReference {

 public static void main(String[] args) throws IOException {
  M m = new M();
  m = null;
  System.gc();

  // 如果不写 main方法退出。System.gc()在垃圾回收线程里；
  // 有可能还没来得及回收main方法就退出了
  System.in.read();
 }
}

public class Soft {

  public static void main(String[] args) {
    System.out.println(Runtime.getRuntime().totalMemory() / 1024 / 1024);
    //10m
    SoftReference<byte[]> m = new SoftReference<>(new byte[1024 * 1024 * 10]);

    System.out.println(m.get());
    System.gc();
    try {
      Thread.sleep(1500);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    System.out.println(m.get());

    // 再分配一个数组，heap装不下去，这时候系统会垃圾回收，
    // 先回收一次，如果不够，会把软引用回收
    byte[] b = new byte[1024 * 1024 * 11];

    System.out.println(m.get());
  }
}

public class Weak {

  public static void main(String[] args) {
    WeakReference<M> m = new WeakReference<>(new M());

    System.out.println(m.get());
    System.gc();
    System.out.println(m.get());
  }
}

public class Phantom {

  private static final List<Object> LIST = new LinkedList<>();
  private static final ReferenceQueue<M> QUEUE = new ReferenceQueue<>();

  public static void main(String[] args) {
    PhantomReference<M> phantomReference = new PhantomReference<>(new M(), QUEUE);

    new Thread(
        () -> {
          while (true) {
            LIST.add(new byte[1024 * 1024]);
            try {
              Thread.sleep(1000);
            } catch (InterruptedException e) {
              Thread.currentThread().interrupt();
            }
            System.out.println(phantomReference.get());
          }

        }

    ).start();

    new Thread(() -> {
      while (true) {
        Reference<? extends M> poll = QUEUE.poll();
        if (poll != null) {
          System.out.println("-------虚引用对象被jvm回收了------" + poll);
        }
      }
    }).start();
  }
}