砍材农夫

强引用

常见的对象赋值引用，不为空，则不会被垃圾回收器回收直到内存溢出

demo

/**
 * 强引用
 */
@Test
public void dd(){
    Object o = new Object();
    System.gc();
    assert o != null;
    o = null;
    assert o == null;
}

GC可达性检测

Image text

软引用

次于强引用，但比强引用更弱，不会被垃圾回收器回收，但内存溢出时，会被回收

适合缓存，比如图片缓存，文件缓存，数据库连接缓存，缓存数据，图片等

    /**
     * 软引用
     */
    @Test
    public void dd1(){
        SoftReference<Object> o = new SoftReference(new Object());
        assert o != null;
    }

弱引用

弱引用，比软引用更弱只能生存到下一次垃圾收集发生之前

ThreadLocalMap 源码结构

    static class ThreadLocalMap {

        /**
         * The entries in this hash map extend WeakReference, using
         * its main ref field as the key (which is always a
         * ThreadLocal object).  Note that null keys (i.e. entry.get()
         * == null) mean that the key is no longer referenced, so the
         * entry can be expunged from table.  Such entries are referred to
         * as "stale entries" in the code that follows.
         */
        static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }

    /**
     * 弱引用
     */
    @Test
    public void dd2(){
        WeakReference<Object> o = new WeakReference(new Object());
        assert o != null;
    }

虚引用

资源回收回调

    /**
     * 虚引用
     */
    @Test
    public void dd3(){
        // 创建一个引用队列
        ReferenceQueue<Object> queue = new ReferenceQueue<>();
        

    }