Java引用机制之强引用、软引用、弱引用和虚引用的实战指南

import java.lang.ref.SoftReference;

public class SoftRefDemo {
    public static void main(String[] args) {
        SoftReference<byte[]> softRef = new SoftReference<>(new byte[10 * 1024 * 1024]); // 10MB
        System.out.println("Before GC: " + softRef.get());

        System.gc();
        System.out.println("After GC: " + softRef.get());  // 大概率存在

        // 占用大量内存触发回收
        try {
            byte[] bigData = new byte[100 * 1024 * 1024];
        } catch (Error e) {
            System.out.println("内存不足，软引用对象被回收: " + softRef.get());
        }
    }
}

import java.lang.ref.WeakReference;

public class WeakRefDemo {
    public static void main(String[] args) {
        WeakReference<String> weakRef = new WeakReference<>(new String("hello weak"));
        System.out.println("Before GC: " + weakRef.get());

        System.gc();
        System.out.println("After GC: " + weakRef.get());  // 必定为 null
    }
}

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

public class PhantomRefDemo {
    public static void main(String[] args) throws InterruptedException {
        ReferenceQueue<Object> queue = new ReferenceQueue<>();
        Object obj = new Object();
        PhantomReference<Object> phantomRef = new PhantomReference<>(obj, queue);

        System.out.println("Phantom get(): " + phantomRef.get()); // 永远是 null

        obj = null;
        System.gc();

        Thread.sleep(100);
        System.out.println("引用队列是否有元素: " + (queue.poll() != null));
    }
}

import java.lang.ref.SoftReference;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class SoftCache<K, V> {
    private final Map<K, SoftReference<V>> cache = new ConcurrentHashMap<>();

    public void put(K key, V value) {
        cache.put(key, new SoftReference<>(value));
    }

    public V get(K key) {
        SoftReference<V> ref = cache.get(key);
        return ref == null ? null : ref.get(); // 若被回收则返回 null
    }
}

import java.util.WeakHashMap;

public class WeakHashMapDemo {
    public static void main(String[] args) {
        WeakHashMap<Object, String> map = new WeakHashMap<>();
        Object key = new Object();
        map.put(key, "value");

        System.out.println("Before: " + map);

        key = null;
        System.gc();

        try { Thread.sleep(100); } catch (Exception ignored) {}
        System.out.println("After: " + map);
    }
}