11 Multithreading and Locks

January 13, 2024 · 3 min read

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11.1 Multithreading #

11.1.1 Implementing Multithreading #

Approach 1: Extend the Thread class.

class Worker extends Thread {
    @Override
    public void run() {
        for (int i = 0; i < 10; i ++ ) {
            System.out.println("Hello! " + this.getName());
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

public class Main {
    public static void main(String[] args) {
        Worker worker1 = new Worker();
        Worker worker2 = new Worker();
        worker1.setName("thread-1");
        worker2.setName("thread-2");
        worker1.start();
        worker2.start();
    }
}

Approach 2: Implement the Runnable interface.

class Worker1 implements Runnable {
    @Override
    public void run() {
        for (int i = 0; i < 10; i ++ ) {
            System.out.println("Hello! " + "thread-1");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

class Worker2 implements Runnable {
    @Override
    public void run() {
        for (int i = 0; i < 10; i ++ ) {
            System.out.println("Hello! " + "thread-2");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

public class Main {
    public static void main(String[] args) {
        new Thread(new Worker1()).start();
        new Thread(new Worker2()).start();
    }
}

11.1.2 Common APIs #

start(): Start a thread.
Thread.sleep(): Put a thread to sleep.
join(): Wait for a thread to finish.
interrupt(): Interrupt a sleeping thread.
SetDaemon (): Set a thread as a daemon thread. When only daemon threads remain, the program exits automatically.

11.2 Locks #

lock: Acquire the lock; block if another thread already holds it.
Unlock: Release the lock and wake other threads blocked on it.

import java.util.concurrent.locks.ReentrantLock;
class Worker extends Thread {
    public static int cnt = 0;
    private static final ReentrantLock lock = new ReentrantLock();


    @Override
    public void run() {
        for (int i = 0; i < 100000; i ++ ) {
            lock.lock();
            try {
	            // Release the lock only after the operation completes
                cnt ++ ;
            } finally {
                lock.unlock();
            }
        }
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Worker worker1 = new Worker();
        Worker worker2 = new Worker();

        worker1.start();
        worker2.start();
        worker1.join();
        worker2.join();

        System.out.println(Worker.cnt);
    }
}

11.3 Synchronization (Synchronized) #

Syntactic sugar for a lock; a shorthand way to use one.

Approach 1: Apply synchronized to a code block.

class Count {
    public int cnt = 0;
}

class Worker extends Thread {
    public final Count count;

    public Worker(Count count) {
        this.count = count;
    }

    @Override
    public void run() {
        synchronized (count) {
            for (int i = 0; i < 100000; i ++ ) {
                count.cnt ++ ;
            }
        }
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Count count = new Count();

        Worker worker1 = new Worker(count);
        Worker worker2 = new Worker(count);

        worker1.start();
        worker2.start();
        worker1.join();
        worker2.join();

        System.out.println(count.cnt);
    }
}

Approach 2: Apply synchronized to a method (locking the this object).

class Worker implements Runnable {
    public static int cnt = 0;

    private synchronized void work() {
        for (int i = 0; i < 100000; i ++ ) {
            cnt ++ ;
        }
    }

    @Override
    public void run() {
        work();
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Worker worker = new Worker();
        Thread worker1 = new Thread(worker);
        Thread worker2 = new Thread(worker);

        worker1.start();
        worker2.start();
        worker1.join();
        worker2.join();

        System.out.println(Worker.cnt);
    }
}

11.3.1 wait and notify #

package org.timerring;

class Worker extends Thread {
    private final Object object;
    private final boolean needWait;

    public Worker(Object object, boolean needWait) {
        this.object = object;
        this.needWait = needWait;
    }

    @Override
    public void run() {
        synchronized (object) {
            try {
                if (needWait) {
                    object.wait();
                    System.out.println(this.getName() + ": 被唤醒啦！");
                } else {
                    object.notifyAll();
                }
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Object object = new Object();
        for (int i = 0; i < 5; i ++ ) {
            Worker worker = new Worker(object, true);
            worker.setName("thread-" + i);
            worker.start();
        }

        Worker worker = new Worker(object, false);
        worker.setName("thread-" + 5);
        Thread.sleep(1000);
        worker.start();
    }
}

11 Multithreading and Locks

January 13, 2024 · 3 min read

11.1 Multithreading #

11.1.1 Implementing Multithreading #

11.1.2 Common APIs #

11.2 Locks #

11.3 Synchronization (Synchronized) #

11.3.1 wait and notify #

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