脚踏实地

/**
 * Runs the garbage collector.
 * <p>
 * Calling the <code>gc</code> method suggests that the Java Virtual
 * Machine expend effort toward recycling unused objects in order to
 * make the memory they currently occupy available for quick reuse.
 * When control returns from the method call, the Java Virtual
 * Machine has made a best effort to reclaim space from all discarded
 * objects.
 * <p>
 * The call <code>System.gc()</code> is effectively equivalent to the
 * call:
 * <blockquote><pre>
 * Runtime.getRuntime().gc()
 * </pre></blockquote>
 *
 * @see     java.lang.Runtime#gc()
 */
public static void gc() {
    Runtime.getRuntime().gc();
}

// 在线程不忙的情况下，GC几乎都会执行都会调用finalize()方法 多试几次(15~30)
public class MainTest {
    public static void main(String[] args) {
        new MainTest();

        // 建议垃圾回收器执行垃圾收集行为 不会立即执行
        System.gc();

        // 调用System.gc();后再调用System.runFinalization(); 会强制调用失去引用对象的 finalize() 方法
//        System.runFinalization();
    }

    @Override
    protected void finalize() throws Throwable {
        super.finalize();
        System.out.println("finalize 被调用 ...");
    }
}

// 强引用测试
public class MainTest {
    public static void main(String[] args) {
        StringBuffer var0 = new StringBuffer("hello world");
        StringBuffer var1 = var0;

        var0 = null;
        System.gc();
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(var1.toString());
    }
}

/**
 * 软引用测试
 * 
 * 虚拟机参数：
 * -Xms10m
 * -Xmx10m
 * -XX:+PrintGCDetails
 */
public class MainTest {
    public static void main(String[] args) {
        //SoftReference<User> softReference = new SoftReference<>(new User("hello"));
        // 上面的一行代码等价于下面的三行代码
        User user = new User("hello");
        SoftReference<User> softReference = new SoftReference<User>(user);
        // 一定要销毁强引用对象 否则创建软引用对象将毫无意义
        user = null;
        System.out.println("创建大对象之前：" + softReference.get());
        try{
            // 模拟堆内存资源紧张 看软引用对象是否会被回收
            byte[] bytes = new byte[1024 * 1024 *7];
        }catch (Throwable e) {
            e.printStackTrace();
        }finally {
            System.out.println("创建大对象之后：" + softReference.get());
        }
    }
}

class User {
    private String name;

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

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                '}';
    }
}

/**
 * 弱引用测试
 */
public class MainTest {
    public static void main(String[] args) {
        WeakReference<User> weakReference = new WeakReference<>(new User("hello"));
        System.out.println("建议GC之前：" + weakReference.get());
        System.gc();
        System.out.println("建议GC之后：" + weakReference.get());
    }
}

class User {
    private String name;

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

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                '}';
    }
}

/**
 * 虚引用测试
 */
public class MainTest {
    // 当前类对象的声明
    public static MainTest obj;
    // 引用队列
    static ReferenceQueue<MainTest> phantomQueue = null;

    @Override
    protected void finalize() throws Throwable {
        super.finalize();
        System.out.println("调用当前类的finalize方法");
        obj = this;
    }

    public static void main(String[] args) {
        Thread thread = new Thread(() -> {
            while(true) {
                if (phantomQueue != null) {
                    PhantomReference<MainTest> objt = null;
                    try {
                        objt = (PhantomReference<MainTest>) phantomQueue.remove();
                    } catch (Exception e) {
                        e.getStackTrace();
                    }
                    if (objt != null) {
                        System.out.println("追踪垃圾回收过程：PhantomReferenceTest实例被GC了");
                    }
                }
            }
        }, "t1");
        thread.setDaemon(true);
        thread.start();

        phantomQueue = new ReferenceQueue<>();
        obj = new MainTest();
        // 构造了PhantomReferenceTest对象的虚引用，并指定了引用队列
        PhantomReference<MainTest> phantomReference = new PhantomReference<>(obj, phantomQueue);
        try {
            System.out.println(phantomReference.get());
            // 去除强引用
            obj = null;
            // 第一次进行GC，由于对象可复活，GC无法回收该对象
            System.out.println("第一次GC操作");
            System.gc();
            Thread.sleep(1000);
            if (obj == null) {
                System.out.println("obj 是 null");
            } else {
                System.out.println("obj 不是 null");
            }
            System.out.println("第二次GC操作");
            obj = null;
            System.gc();
            Thread.sleep(1000);
            if (obj == null) {
                System.out.println("obj 是 null");
            } else {
                System.out.println("obj 不是 null");
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {

        }
    }
}