ThreadLocal相关代码和面试题

1代码1

class House {
    int saleCount = 0;

    public synchronized void saleHouse() {
        saleCount++;
    }

//    ThreadLocal<Integer> saleVolume = new ThreadLocal<Integer>() {
//        @Override
//        protected Integer initialValue() {
//            return 0;
//        }
//    }; 

    ThreadLocal<Integer> saleVolume = ThreadLocal.withInitial(() -> 0);

    public void saleVolumeByThreadLocal() {
        saleVolume.set(1 + saleVolume.get());
    }
}

public class ThreadLocalDemo {

    public static void main(String[] args) {

        House house = new House();

        for (int i = 1; i <= 5; i++) {
            try {
                new Thread(() -> {
                    int size = new Random().nextInt(5) + 1;
                    // System.out.println(size);
                    for (int j = 1; j <= size; j++) {
                        house.saleHouse();
                        house.saleVolumeByThreadLocal();
                    }
                    System.out.println(Thread.currentThread().getName() + "\t" + "卖出多少套" + house.saleVolume.get());

                }, String.valueOf(i)).start();
            }catch (Exception e) {

            }finally {
                house.saleVolume.remove();
            }
        }

        try {
            TimeUnit.MILLISECONDS.sleep(300);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println(Thread.currentThread().getName() + "\t" + "共卖出多少套:" + house.saleCount);

    }
}

2.代码2

class MyData {

    ThreadLocal<Integer> threadLocalField = ThreadLocal.withInitial(() -> 0);

    public void add() {
        threadLocalField.set(1 + threadLocalField.get());
    }
}

public class ThreadLocalDemo1 {

    public static void main(String[] args) {
        ExecutorService threadPool = Executors.newFixedThreadPool(3);
        MyData myData = new MyData();

        try {
            for (int i = 0; i < 10; i++) {
                threadPool.submit(() -> {
                    try {
                        Integer beforeInt = myData.threadLocalField.get();
                        myData.add();
                        Integer afterInt = myData.threadLocalField.get();
                        System.out.println(Thread.currentThread().getName() + "\t" + "beforeInt " + beforeInt + "\t" + " afterInt " + afterInt);
                    } finally {
                        myData.threadLocalField.remove();
                    }
                });
            }
        } finally {
            threadPool.shutdown();
        }

        try {
            TimeUnit.MILLISECONDS.sleep(300);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }
}

3.强软弱虚四大应用代码:

class MyObject {

    //这个方法一般不用复写
    @Override
    protected void finalize() throws Throwable {
        System.out.println("-----invoke finalize method---");
    }
}
public class ReferenceDemo {
    public static void main(String[] args) {
        MyObject myObject = new MyObject();
        ReferenceQueue<MyObject> referenceQueue = new ReferenceQueue<>();
        PhantomReference<MyObject> phantomReference = new PhantomReference<>(myObject, referenceQueue);
        List<byte[]> list = new ArrayList<>();
        new Thread(() ->{
            while (true) {
                list.add(new byte[10 * 1024 * 1024]);
                try {
                    TimeUnit.MILLISECONDS.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(phantomReference.get()+"\t" +"list add ok");
            }
        },"t1").start();

       new Thread(() -> {
           while (true) {
               Reference<? extends MyObject> reference = referenceQueue.poll();
               if(reference != null) {
                   System.out.println("有虚对象回收加入了队列");
                   break;
               }
           }
       },"t2").start();
    }

    private static void strongReference() {
        MyObject myObject = new MyObject();
        System.out.println("gc beore: " +myObject);
        myObject = null;
        System.gc(); //人工开启gc,一般不用
        try {
            TimeUnit.MILLISECONDS.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("gc after: " + myObject);
    }

    private static void softReference() {
        SoftReference<MyObject> softReference = new SoftReference<>(new MyObject());

        System.gc(); //人工开启gc,一般不用
        System.out.println("gc after 内存够用: " + softReference.get());
        try {
            TimeUnit.MILLISECONDS.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        try {
            byte[] bytes = new byte[50 * 1024 * 1024];
        }catch (Exception e) {
            e.printStackTrace();
        } finally {
            System.out.println("gc after 内存不够: " + softReference.get());
        }

    }

    private static void weakReference() {
        WeakReference<MyObject> weakReference = new WeakReference<>(new MyObject());
        System.out.println("gc beore: " +weakReference.get());
        System.gc(); //人工开启gc,一般不用
        try {
            TimeUnit.MILLISECONDS.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("gc after: " + weakReference.get());
    }
}

 

 

5.为什么要用弱引用?不用会怎么样