How to solve common problems with Java thread status

To master the common problems and solutions of Java thread status, specific code examples are required

In Java multi-thread programming, thread status is an important concept. Understanding and mastering thread status can not only help us better understand how multi-threading works, but also help us solve some common threading problems. This article will introduce several common thread status problems and their solutions, and provide corresponding code examples.

1. Five states of threads
   In Java, threads have five states: NEW, RUNNABLE, BLOCKED, WAITING, TERMINATED. Among them, the NEW state indicates that the thread has been created but has not yet started execution, the RUNNABLE state indicates that the thread is executing or waiting to be executed, the BLOCKED state indicates that the thread is blocked due to competition for shared resources, the WAITING state indicates that the thread is waiting for other threads to wake up, and the TERMINATED state Indicates that the thread has completed execution or was terminated early.
2. Thread deadlock
   Thread deadlock is a classic thread state problem. Deadlock occurs when multiple threads wait for each other to release resources. The following is a simple example:

public class DeadlockExample {
    private static Object resource1 = new Object();
    private static Object resource2 = new Object();
  
    public static void main(String[] args) {
        // 线程1
        Thread thread1 = new Thread(() -> {
            synchronized (resource1) {
                System.out.println("Thread 1: Holding resource 1");
            
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            
                synchronized (resource2) {
                    System.out.println("Thread 1: Holding resource 1 and resource 2");
                }
            }
        });
    
      // 线程2
        Thread thread2 = new Thread(() -> {
            synchronized (resource2) {
                System.out.println("Thread 2: Holding resource 2");
            
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            
                synchronized (resource1) {
                    System.out.println("Thread 2: Holding resource 1 and resource 2");
                }
            }
        });
    
        // 启动线程
        thread1.start();
        thread2.start();
    }
}

In the above code, two threads hold resource1 and resource2 resources respectively, and try to obtain the resources held by each other at the same time. When running this program, a deadlock occurs, preventing the program from ending normally.

The way to solve the deadlock problem is to avoid waiting for resources in a loop. For example, you can obtain resources in a fixed order, or set a timeout mechanism to give up competing for resources.

3. Thread insecurity issue
   In a multi-threaded environment, operations on shared resources may lead to data inconsistency. This is the issue of thread insecurity. The following is a simple example:

public class ThreadUnsafeExample {
    private static int count = 0;
  
    public static void main(String[] args) {
        Thread thread1 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                count++;
            }
        });
    
        Thread thread2 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                count++;
            }
        });
    
        thread1.start();
        thread2.start();
    
        try {
            thread1.join();
            thread2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    
        System.out.println("Count: " + count);
    }
}

In the above code, the two threads perform cumulative operations on the global variable count. Due to the uncertainty of the thread execution order, the final calculated count value is uncertain, which may lead to incorrect results.

The way to solve the problem of thread insecurity is to use synchronization mechanism, such as using the synchronized keyword or Lock lock to protect shared resources.

4. Thread waiting and waking up
   Thread waiting and waking up is another common thread state problem. The following is a simple example:

public class WaitNotifyExample {
    private static Object lock = new Object();

    public static void main(String[] args) {
        Thread thread1 = new Thread(() -> {
            synchronized (lock) {
                try {
                    System.out.println("Thread 1: Waiting for lock");
                    lock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
    
                System.out.println("Thread 1: Resumed");
            }
        });

        Thread thread2 = new Thread(() -> {
            synchronized (lock) {
                System.out.println("Thread 2: Acquired lock");
    
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
    
                lock.notify();
                System.out.println("Thread 2: Notified");
            }
        });
    
        thread1.start();
        thread2.start();
    }
}

In the above code, thread 1 acquires the lock first and enters the waiting state until thread 2 calls the notify() method to wake up thread 1. When thread 1 is awakened, it will continue to execute the following code.

The way to solve the problem of thread waiting and waking up is to use the wait() and notify() methods in the Object class to cooperate between threads.

To sum up, mastering the common problems and solutions of Java thread status is crucial to understanding multi-threaded programming. Through the above code examples, we can better understand and apply thread status and avoid common threading problems in actual multi-threaded development.

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