synchronized 读写同步

synchronized 读写同步

这是一道面试题，很多人也遇到了。

要求：1.读-读 不用线程同步。2.读-写 要求线程同步，写的时候不能读。3.写-写同步。写的时候不能写。

java lock读写锁是好的处理方案。这里不说了。但人家问的是synchronized 读写同步。

假设两个方法，write() ,read();

1. read()之间不需要同步，正常的就行。
2. read()与write()之间需要同步。
3. write()之间需要同步。

1.和3.很好理解也很现实。

经过思考：write()必须是同步的，所有对象都要同步。那这里需要设计成静态的。如果不静态的，保证每次同步都是一个锁。
其次 read（） 之前要栓查write同步锁，但又没有对应方法可以检查。想到的是 再写一个栓查方法。这个方法与write使用同一个同步锁（设计在同一个类，同时静态）。但这个方法是空的，什么也不做。
所以设计成如下代码：   

package test;

public class Test {

    public void read() {

        canRead();
        System.out.println(Thread.currentThread().getName() + "-->>begain read");
        System.out.println(Thread.currentThread().getName() + "-->>reading");
        try {
            Thread.sleep(1100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + "-->>read end");
    }

    public static synchronized void canRead() {

    }

    public static synchronized void write() {
        System.out.println(Thread.currentThread().getName() + "-->>begain write");
        canRead();
        System.out.println(Thread.currentThread().getName() + "-->>writing");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + "-->>writing end");
        return;
    }

    public static void main(String[] args) {

        Test test = new Test();
        Thread t = new Thread(new Runnable() {

            @Override
            public void run() {
                while (true) {
                    test.read();
                    try {
                        Thread.sleep(500);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        });
        t.setName("read_thread_1");
        t.start();

        Thread t1 = new Thread(new Runnable() {

            @Override
            public void run() {
                while (true) {
                    test.read();
                    try {
                        Thread.sleep(500);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        });
        t1.setName("read_thread_2");
        t1.start();

        Thread t2 = new Thread(new Runnable() {

            @Override
            public void run() {
                while (true) {
                    try {
                        Thread.sleep(3000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    Test.write();
                }
            }
        });
        t2.setName("write_thread");
        t2.start();
        Thread t3 = new Thread(new Runnable() {

            @Override
            public void run() {
                while (true) {
                    try {
                        Thread.sleep(3000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    Test.write();
                }
            }
        });
        t3.setName("write_thread_2");
        t3.start();

    }

}

运行结果如下，通过结果分析，读线程是穿插的，不存在同步。写线程都是完整执行过程，是同步的。

read_thread_1-->>begain read

read_thread_1-->>reading

read_thread_2-->>begain read

read_thread_2-->>reading

read_thread_1-->>read end

read_thread_2-->>read end

read_thread_1-->>begain read

read_thread_1-->>reading

read_thread_2-->>begain read

read_thread_2-->>reading

read_thread_1-->>read end

read_thread_2-->>read end

write_thread-->>begain write

write_thread-->>writing

write_thread-->>writing end

read_thread_2-->>begain read

read_thread_2-->>reading

read_thread_1-->>begain read

read_thread_1-->>reading

write_thread_2-->>begain write

write_thread_2-->>writing

write_thread_2-->>writing end

read_thread_2-->>read end

read_thread_1-->>read end

read_thread_2-->>begain read

read_thread_2-->>reading

read_thread_1-->>begain read

read_thread_1-->>reading

read_thread_2-->>read end

read_thread_1-->>read end

write_thread-->>begain write

write_thread-->>writing

write_thread-->>writing end

write_thread_2-->>begain write

write_thread_2-->>writing

write_thread_2-->>writing end

read_thread_1-->>begain read

read_thread_1-->>reading

read_thread_2-->>begain read

read_thread_2-->>reading

read_thread_1-->>read end

read_thread_2-->>read end

read_thread_1-->>begain read

read_thread_1-->>reading

read_thread_2-->>begain read

read_thread_2-->>reading

write_thread-->>begain write

write_thread-->>writing

read_thread_1-->>read end

read_thread_2-->>read end

write_thread-->>writing end

write_thread_2-->>begain write

write_thread_2-->>writing

write_thread_2-->>writing end

read_thread_2-->>begain read

read_thread_2-->>reading

read_thread_1-->>begain read

read_thread_1-->>reading

read_thread_2-->>read end

read_thread_1-->>read end

read_thread_2-->>begain read

read_thread_1-->>begain read

read_thread_1-->>reading

read_thread_2-->>reading

write_thread-->>begain write

write_thread-->>writing

这个代码冗余部分是要执行一个空方法，目前还没有想到其它方法，有大牛有好办法可以留言互相学习。

代码 bug :开始写的时候还没有读完的线程。会导致错误。

-----待改进-----------