引用与垃圾回收

import static org.junit.Assert.assertNotNull;
import static org.junit.Assert.assertNull;

import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;

import org.junit.Test;

public class ReferenceTest {

    @Test
    public void strongReference() {

        Dog ref0 = new Dog("oooo");
        // 加入新引用
        Dog ref1 = ref0;
        // 断开ref0
        ref0 = null;

        System.gc();
        assertNotNull(ref1.getName());
    }

    /**
     * -Xmx32m
     */
    @Test
    public void softReferenceCase1() {
        // 占用过半内存
        byte[] ref0 = new byte[1024 * 1024 * 17];
        // 加入新引用（soft)
        SoftReference<byte[]> ref1 = new SoftReference<byte[]>(ref0);
        // 断开ref0
        ref0 = null;
        // 如果内存不够用，则ref1 被回收
        byte[] ref2 = new byte[1024 * 1024 * 17];
        assertNull(ref1.get());
        assertNotNull(ref2);
    }

    @Test
    public void softReferenceCase2() {

        Dog ref0 = new Dog("dddd");

        // 加入新引用（soft)
        SoftReference<Dog> ref1 = new SoftReference<Dog>(ref0);
        // 断开ref0
        ref0 = null;

        System.gc();
        System.gc();
        assertNotNull(ref1.get().getName());
    }

    @Test
    public void weakReferenceCase1() {
        Dog ref0 = new Dog("ssss");

        // 加入新引用（weak)
        WeakReference<Object> ref1 = new WeakReference<Object>(ref0);
        // 断开ref0
        ref0 = null;

        System.gc();
        assertNull(ref1.get());
    }

    @Test
    public void weakReferenceCase2() {
        Dog ref0 = new Dog("gggg");

        // 加入新引用（weak)
        WeakReference<Dog> ref1 = new WeakReference<Dog>(ref0);

        System.gc();
        // 存在强引用无法回收
        assertNotNull(ref1.get());

        // 断开ref0
        ref0 = null;

        System.gc();
        assertNull(ref1.get());
    }

    @Test
    public void phantomReference() {
        Object ref0 = new Object();
        // 加入新引用（phantom)
        ReferenceQueue<Object> queue = new ReferenceQueue<Object>();
        PhantomReference<Object> ref1 = new PhantomReference<Object>(ref0, queue);
        // this method always returns null.
        assertNull(ref1.get());
    }

    class Dog {

        private String name;

        public Dog(String name) {
            this.name = name;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

    }
}