Java的四种引用方式（强、软、弱、虚引用）

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//创建一个用来测试的类
public class ReferenceDemo {
    private String name;
    private static final int SIZE = 1000;
    private Long[] longArray = new Long[SIZE];
    public ReferenceDemo(String name){
        this.name = name;
    }
    public ReferenceDemo(String name, int size) {
    this.name = name;
    longArray = new Long[size];
}
}

欢迎大家来到IT世界,在知识的湖畔探索吧!

欢迎大家来到IT世界,在知识的湖畔探索吧!class ReferenceTest{
    public static void main(String[] args) throws InterruptedException {
        System.out.println("start test strongReference");
        ReferenceDemo reference = new ReferenceDemo("strongReference");
        //通过给strongReference赋值，创建一个强引用
        Object strongReference = reference;
        //证明reference和strongReference是同一个对象
        System.out.println(reference == strongReference);
        //给reference赋值为null，也就是说reference变量不指向对象
        reference = null;
        //当把strongReference也指向null时，此时new ReferenceDemo("strongReference");
        //没有任何引用，因此就会被垃圾回收器给回收
        //strongReference = null;
        //手动触发GC
        System.gc();
        Thread.sleep(500);
        //由于对象还存在引用，所以在触发GC后对象未被回收
        System.out.println(strongReference);
        System.out.println("end test strongReference");
    }
}

输出结果：

start test strongReference
true
ReferenceDemo@8efb846
end test strongReference

欢迎大家来到IT世界,在知识的湖畔探索吧!class ReferenceTest{
    public static void main(String[] args) throws InterruptedException {
     SoftReference<ReferenceDemo> softReference = new SoftReference<>(new ReferenceDemo("softReference"));
    System.out.println(softReference.get());
    System.gc();
    Thread.sleep(500);
    //即使手动触发后，对象仍然不会被回收
    System.out.println(softReference.get());
    int i = 0;
    while (true) {
        try {
            ++i;
            new ReferenceDemo("oom ", 400_000_000);
        } catch (Throwable e) {
            System.out.println("OOM after " + i + " times");
            e.printStackTrace();
            break;
        }
    }
    //当发生内存溢出时，对象被回收
    System.out.println(softReference.get());
    }
}

测试结果：

com.jinlei.juc.juc.T_001.lock.ReferenceDemo@8efb846
com.jinlei.juc.juc.T_001.lock.ReferenceDemo@8efb846
OOM after 1 times
java.lang.OutOfMemoryError: Java heap space
	at com.jinlei.juc.juc.T_001.lock.ReferenceDemo.<init>(ReferenceDemo.java:16)
	at com.jinlei.juc.juc.T_001.lock.ReferenceTest.main(ReferenceDemo.java:49)
null

class ReferenceTest {
    public static void main(String[] args) throws InterruptedException {
        System.out.println("start test weakReference");
        //创建一个弱引用对象
        WeakReference<ReferenceDemo> weakReference = new WeakReference<>(new ReferenceDemo("weakReference"));
        System.out.println(weakReference.get());
        //ReferenceDemo strongReference = weakReference.get();    7
        //手动触发GC
        System.gc();
        Thread.sleep(500);
        System.out.println(weakReference.get());
        System.out.println("end test weakReference");
    }
}

测试结果：

start test weakReference
com.jinlei.juc.juc.T_001.lock.ReferenceDemo@8efb846
null
end test weakReference

当有强引用，第7行代码打开时，对象是不会被回收的：

start test weakReference
com.jinlei.juc.juc.T_001.lock.ReferenceDemo@8efb846
com.jinlei.juc.juc.T_001.lock.ReferenceDemo@8efb846
end test weakReference

//创建一个引用队列
ReferenceQueue<Object> referenceQueue = new ReferenceQueue<>();
//创建一个虚引用
PhantomReference<ReferenceDemo> phantomReference = new PhantomReference<>(new ReferenceDemo("phantomReference"),referenceQueue);
//虚引用的get方法永远返回为null
System.out.println(phantomReference.get());
System.out.println(referenceQueue.poll());
System.gc();
Thread.sleep(500);
System.out.println(referenceQueue.poll());

测试结果如下：

null
null
java.lang.ref.PhantomReference@8efb846