Java后端面试，强、软、弱、幻象引用的概念以及使用场景

java复制代码public class StrongReferenceExample {
    public static void main(String[] args) {
        Object strongRef = new Object(); // strongRef 是一个强引用
        // 只要 strongRef 存在，对象不会被垃圾回收
    }
}

java复制代码import java.lang.ref.SoftReference;

public class SoftReferenceExample {
    public static void main(String[] args) {
        Object obj = new Object();
        SoftReference<Object> softRef = new SoftReference<>(obj);
        obj = null; // 允许 obj 被垃圾回收

        // 在内存不足时，softRef 引用的对象会被回收
        if (softRef.get() == null) {
            System.out.println("Object has been garbage collected");
        } else {
            System.out.println("Object is still available");
        }
    }
}

java复制代码import java.lang.ref.WeakReference;

public class WeakReferenceExample {
    public static void main(String[] args) {
        Object obj = new Object();
        WeakReference<Object> weakRef = new WeakReference<>(obj);
        obj = null; // 允许 obj 被垃圾回收

        // 在垃圾回收器运行时，weakRef 引用的对象会被回收
        if (weakRef.get() == null) {
            System.out.println("Object has been garbage collected");
        } else {
            System.out.println("Object is still available");
        }
    }
}

java复制代码import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;

public class PhantomReferenceExample {
    public static void main(String[] args) {
        Object obj = new Object();
        ReferenceQueue<Object> refQueue = new ReferenceQueue<>();
        PhantomReference<Object> phantomRef = new PhantomReference<>(obj, refQueue);
        obj = null; // 允许 obj 被垃圾回收

        // 在垃圾回收器运行并回收对象后，phantomRef 会被加入到引用队列中
        if (refQueue.poll() != null) {
            System.out.println("Object has been garbage collected and phantom reference is enqueued");
        } else {
            System.out.println("Object is still available or garbage collector hasn't run yet");
        }
    }
}