Basic usage of synchronized locks

1. Decorated code block , Specify the lock object , Lock a given object , Get the lock of the given object before entering the synchronization code block , And it needs to be the same object lock , That is, create an implementation Runnable The interface needs to be the same object , If two objects are created, they do not belong to the same object lock ：

public class ThreadCount implements Runnable{
    private static int count = 100;

    @Override
    public void run() {
        while (true){
            cal();
        }
    }

    private void cal() {
        if (count > 1){
            //  Lock 
            synchronized (this){
                count--;
                System.out.println(Thread.currentThread().getName() + " " + count);
            }
        }
    }

    public static void main(String[] args) {
        ThreadCount threadCount = new ThreadCount();
        new Thread(threadCount).start();
        new Thread(threadCount).start();
    }
}

2. Decorated instance method , Locks on the current instance , Obtain the lock for the current instance before entering the synchronized code ：

package thread;

public class ThreadCount implements Runnable{
    private static int count = 100;

    @Override
    public void run() {
        while (true){
            cal();
        }
    }

    //  If you will synchronized When added to the instance method, use this lock ,  Equivalent to the previous writing ,  Therefore, it is generally more convenient to lock the method 
    private synchronized void cal() {
        if (count > 1){
            count--;
            System.out.println(Thread.currentThread().getName() + " " + count);
        }
    }

    public static void main(String[] args) {
        ThreadCount threadCount = new ThreadCount();
        new Thread(threadCount).start();
        new Thread(threadCount).start();

    }
}

3. If you will synchronized Add to static methods , The current class name is used by default .class：

package thread;

public class ThreadCount implements Runnable{
    private static int count = 100;

    @Override
    public void run() {
        while (true){
            cal();
        }
    }

    //  If you will synchronized Add to static methods , The current class name is used by default .class
    private void cal() {
        synchronized(ThreadCount.class){
            if (count > 1){
                count--;
                System.out.println(Thread.currentThread().getName() + " " + count);
            }
        }
    }

    public static void main(String[] args) {
        ThreadCount threadCount = new ThreadCount();
        new Thread(threadCount).start();
        new Thread(threadCount).start();
    }
}

4.  If we are using synchronized We need to pay attention to synchronized The problem of lock nesting avoids the occurrence of deadlock ：