并发编程之美读书笔记 1

线程的概念

线程不能独立存在,资源由操作系统分配给进程,但是CPU资源是分配给线程的.同一个进程内的线程:

共享: 堆(主要存放使用new操作创建的对象实例)和方法区(JVM加载的类,常量以及静态变量等)
独立: 程序计数器(获得CPU控制权后可以从计数器指定地址继续执行)和栈区域.

当启动main函数时,就相当于启动了一个JVM的进程,而main所在的线程就是该进程中的主线程.

如何创建线程

有三种方式:

继承Thread类,重写run方法:

// Thread1.java
public class Thread1 extends Thread{

    @Override
    public void run() {
        System.out.println("current-thread: "+this.getName());
        System.out.println("继承自Thread");
    }

    public static void main(String[] args) {
        Thread1 t = new Thread1();
        t.start();
    }
}

不需要使用currentThread就可以获取当前线程,但是不能再继承其他类

实现 Runnable方法,并重写run方法:

//Thread2.java
public class Thread2 implements Runnable{
    private String name;

    public Thread2(String name) {
        this.name = name;
    }

    @Override
    public void run() {
        System.out.println("this is "+ name + " running");
        System.out.println("实现了Runnable接口");
    }

    public static void main(String[] args) {
        Thread t1 = new Thread(new Thread2("t1"));
        t1.start();
        Thread t2 = new Thread(new Thread2("t2"));
        t2.start();
    }
}

可以继承其他类,还可以添加参数区分

实现Callable接口的call方法:

//Thread3.java
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;

public class Thread3 implements Callable<String> {
    @Override
    public String call(){
        System.out.println("使用FutureTask方式,可以有返回值");
        return "done";
    }

    public static void main(String[] args) throws InterruptedException {
        FutureTask<String> ft = new FutureTask<String>(new Thread3());
        Thread t = new Thread(ft);
        t.start();
        try {
            String result = ft.get();
            System.out.println(result);
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }
}

具有返回值,可以使用get方法获得返回值.也可以继承其他类.

当调用Thread的start方法,线程只是进入了就绪状态,只有当cpu资源得到满足时,才会真正开始运行.run方法结束后,线程就处于终止状态.

wait与notify

wait与notify都是Object类中的方法.

wait

只有获得了变量的监视锁,才能调用该变量的wait方法,否则会抛出异常:

public class WaitExample {
    private int a;

    void incr() {
        a++;
    }

    int getA() {
        return a;
    }
    public static void main(String[] args) {
        WaitExample waitExample = new WaitExample();
        try {
            // wrong
            waitExample.wait();
            for (int i = 0; i < 20000; i++) {
                Thread t = new Thread(new Runnable() {
                    @Override
                    public void run() {
                        waitExample.incr();
                    }
                });
                t.start();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

        System.out.println(waitExample.getA());

    }
}

运行结果:

java.lang.IllegalMonitorStateException
    at java.lang.Object.wait(Native Method)
    at java.lang.Object.wait(Object.java:502)
    at com.may.learning.WaitExample.main(WaitExample.java:17)
0

获得监视锁可以通过synchronized关键字:
比如在方法上加上关键字:

    synchronized void incr() {
        a++;
    }

或者,在调用时对该类加上关键字:

synchronized (waitExample) {
...
}

当调用wait时,只会释放当前变量的锁:

    public static void main(String[] args) throws InterruptedException{
        WaitExample waitExample = new WaitExample();
        WaitExample anotherWaitExample = new WaitExample();
        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (waitExample) {
                    System.out.println("t: get waitExample lock success!");
                    System.out.println("t: try get anotherWaitExample lock");
                    try {
                        synchronized (anotherWaitExample) {
                            System.out.println("t: get anotherWaitExample success!");
                            System.out.println("t: release waitExample lock");
                            // t只释放了waitExample的锁
                            waitExample.wait();

                        }
                    } catch (InterruptedException e) {
                        System.out.println("被中断");
                    }
                }

            }
        });
        t.start();
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (waitExample) {
                    System.out.println("t1: get waitExample lock success!");
                    System.out.println("t1: try get anotherWaitExample lock");
                    try {
                        // 由于t未释放锁,因此t1获取不到anotherWaitExample的锁
                        synchronized (anotherWaitExample) {
                            System.out.println("t1: get anotherWaitExample success!");
                            System.out.println("t1: release waitExample lock");
                            waitExample.wait();

                        }
                    } catch (InterruptedException e) {
                        System.out.println("被中断");
                    }
                }

            }
        });
        t1.start();
        Thread.sleep(1000);
    }

运行结果:

t: get waitExample lock success!
t: try get anotherWaitExample lock
t: get anotherWaitExample success!
t: release waitExample lock
t1: get waitExample lock success!
t1: try get anotherWaitExample lock

如果需要指定时间,可以使用wait(long timeout)或者wait(long timeout, int nanos).
被wait挂起时,被中断时会抛出异常:

public class WaitExample {

    private int a;

    void incr() {
        a++;
    }

    int getA() {
        return a;
    }

    public static void main(String[] args) throws InterruptedException{
        WaitExample waitExample = new WaitExample();
        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (waitExample) {
                    waitExample.incr();
                    try {
                        waitExample.wait();
                    } catch (InterruptedException e) {
                        System.out.println("被中断");
                    }
                }

            }
        });
        t.start();
        Thread.sleep(1000);
        t.interrupt();
        System.out.println(waitExample.getA());
    }
}

运行结果:

1
被中断

notify

notify会唤醒被wait阻塞挂起的线程.与wait类似,需要获得了变量的监视锁后才能对其进行进行notify操作.如果有多个线程都在等待唤醒,那么会随机唤醒某一个线程.而notifyAll会唤醒所有阻塞等待的线程.
类似wait方法，notify也需要先获得对象的监视锁（但是notify不会释放锁）。

    public static void main(String[] args) throws InterruptedException {
        NotifyExample notifyExample = new NotifyExample();
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    synchronized (notifyExample) {
                        System.out.println("t1 挂起");
                        notifyExample.wait();
                        System.out.println("t1 被唤醒了");
                    }
                } catch (InterruptedException e) {
                    System.out.println("被中断了");
                }
            }
        });
        t1.start();
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    synchronized (notifyExample) {
                        System.out.println("t2 挂起");
                        notifyExample.wait();
                        System.out.println("t2 被唤醒了");
                    }
                } catch (InterruptedException e) {
                    System.out.println("被中断了");
                }
            }
        });
        t2.start();
        Thread.sleep(1000);
        synchronized (notifyExample) {
            notifyExample.notify();
        }
    }

运行结果:

t1 挂起
t2 挂起
t1 被唤醒了

如果将notifyExample.notify()换成notifyExample.notifyAll(),那么t1和t2都能被唤醒:

t1 挂起
t2 挂起
t2 被唤醒了
t1 被唤醒了

生产者消费者

利用wait和notify写一个简单的生产者消费者:

import java.util.LinkedList;
import java.util.Queue;

public class ProducerConsumer {

    public static void main(String[] args) throws InterruptedException {
        Queue<Integer> queue = new LinkedList<>();
        Thread producer = new Thread(new Runnable() {
            @Override
            public void run() {
                int i=0;
                synchronized (queue) {
                    while (true){
                        try {
                            if (!queue.isEmpty()) {
                                queue.wait();
                            }else{
                                queue.add(++i);
                                System.out.println("生产了"+i);
                                queue.notify();
                            }
                        } catch (InterruptedException e) {
                            System.out.println("被中断了");
                            return;
                        }
                    }
                }
            }
        });
        Thread consumer = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (queue) {
                    while (true){
                        try{
                            if (queue.isEmpty()) {
                                queue.wait();
                            }else{
                                int i = queue.poll();
                                System.out.println("消费了"+i);
                                queue.notify();
                            }
                        }catch (InterruptedException e){
                            System.out.println("被中断了");
                            return;
                        }
                    }
                }
            }
        });
        producer.start();
        consumer.start();
        Thread.sleep(10000);
        producer.interrupt();
        consumer.interrupt();
    }
}

运行结果:

...
生产了782421
消费了782421
生产了782422
消费了782422
生产了782423
消费了782423
生产了782424
被中断了
被中断了

join

wait与notify属于Ojbect的方法,而join是Thread的方法.
调用某一线程的join方法,可以使当前线程（下例中的main线程）阻塞等待该线程完成:

public class JoinTest {

    public static void main(String[] args) throws InterruptedException {
        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(2000);
                    System.out.println("t 结束了!");
                } catch (InterruptedException e) {
                    System.out.println("被中断了");
                }
            }
        });
        t.start();
        t.join();
        System.out.println("main 结束了!");
    }
}

运行结果:

t 结束了!
main 结束了!

sleep

sleep和wait 一样可以挂起当前线程,被中断时也会抛出异常.但不同的是它是属于Thread的方法,并且它不会释放对象的监视锁.指定睡眠时间到达后,会进入就绪状态,等待cpu的调度.

public class SleepTest {
    public static void main(String[] args) throws InterruptedException {
        SleepTest sleepTest = new SleepTest();
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (sleepTest){
                    try {
                        System.out.println("t1 获取到锁");
                        Thread.sleep(60000);
                    } catch (InterruptedException e) {
                        System.out.println("被中断了!");
                    }
                }
            }
        });
        t1.start();
        Thread.sleep(1000);
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("t2 等待锁");
                synchronized (sleepTest) {
                    System.out.println("t2 获取到锁了");
                }
            }
        });
        t2.start();
        Thread.sleep(2000);
        t1.interrupt();
    }
}

运行结果:

t1 获取到锁
t2 等待锁
被中断了!
t2 获取到锁了

yield

线程会主动让出自己的cpu时间,并不会被挂起,只是进入了就绪状态.但是下一次仍然可能被调度到.

中断

前面已经使用了interrupt很多次,需要注意与isInterrupted以及interrupted进行区分.
isInterrupted: 判断当前线程是否被中断;
interrupted: 判断当前线程是否被中断,并且复位(清除中断标志);

使用interrupted判断中断并且复位:

public class InterruptTest {

    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                while (!Thread.interrupted()) {

                }
                System.out.println("t1 is Interrupted: "+Thread.currentThread().isInterrupted());
            }
        });
        t1.start();
        t1.interrupt();
        t1.join();
        System.out.println("done");
    }
}

运行结果:

t1 is Interrupted: false
done

需要注意的是调用线程的interrupt方法,只是为该线程设置中断位.当线程在运行时,会立即往下执行.而当被wait,join或者sleep方法阻塞挂起时,才会抛出InterruptedException异常,此时中断位会被恢复,如下面这个例子:

public class InterruptTest throws InterruptedException{

    public static void main(String[] args){
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("t1 running");
                while (true) {
                }
            }
        });
        t1.start();
        t1.interrupt();
        System.out.println("main thread isInterrupted: " + Thread.interrupted());
        System.out.println("t1 isInterrupted: " + t1.isInterrupted());
        boolean isInterrupted = Thread.interrupted();
        System.out.println("isInterrupted: " + isInterrupted);
        System.out.println("main thread isInterrupted: " + Thread.interrupted());
        System.out.println("t1 isInterrupted: " + t1.isInterrupted());
        t1.join();
        System.out.println("done");
    }
}

输出结果:

main thread isInterrupted: false
t1 isInterrupted: true
isInterrupted: false
main thread isInterrupted: false
t1 running
t1 isInterrupted: true

因为t1并没有被挂起,所以即使中断位为true,也仍然在继续运行,不会输出"done".而interrupted针对是当前线程,也就是主线程,所以中断标志位一直是false.
如果在阻塞情况下,中断位会被马上复位,对上面的例子稍加修改:

public class InterruptTest {

    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                try{
                    while (!Thread.currentThread().isInterrupted()) {
                        System.out.println("t1: running");
                        Thread.sleep(1000);
                    }
                }catch (Exception e){
                    System.out.println("t1 被中断");
                    System.out.println("thread t1 isInterrupted : "+Thread.currentThread().isInterrupted());
                }
            }
        });
        t1.start();
        Thread.sleep(1000);
        t1.interrupt();
        System.out.println("main thread isInterrupted: " + Thread.interrupted());
        System.out.println("t1 isInterrupted: " + t1.isInterrupted());
        t1.join();
        System.out.println("done");
    }
}

运行结果:

t1: running
t1 被中断
thread t1 isInterrupted : false
main thread isInterrupted: false
t1 isInterrupted: false
done

主线程中的isInterrupted仍然是false,但是打印出的t1的isInterrupted由于处在阻塞状态被中断,中断位又被复位成了false.

上下文切换与死锁

当出现:

CPU时间用尽,线程进入就绪状态时
被其他线程中断时

会出现上下文的切换.

避免死锁的出现,可以采用不同线程都按照相同的顺序去请求资源,保证资源获取的有序一致性.

守护线程与用户线程

守护线程的存在不影响JVM的退出,只要最后一个用户线程没有退出,正常情况下JVM也不会退出.
JVM没有退出:

public class DaemonTest {

    public static void main(String[] args) {
        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {

                }
            }
        });
//        t.setDaemon(true);
        t.start();
        System.out.println("main exit");
    }
}

运行结果:

main exit

将t利用setDeamon设置为守护线程的注释打开,运行结果:

main exit

Process finished with exit code 0

原理是,main运行后,JVM会自动启动一个叫做DestoryJavaVM的线程,它会等待所有用户线程结束后终止JVM进程.

ThreadLocal

如果创建了一个ThreadLocal变量,每个访问这个变量的线程都会获得一个副本.

public class ThreadLocalTest {
    static ThreadLocal<Integer> val = new ThreadLocal<>();

    static void print(String string) {
        System.out.println(string + ":" + val.get());
    }
    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                val.set(1);
                for (int i = 0; i < 50; i++) {
                    int v = val.get();
                    v++;
                    val.set(v);
                }
                print("thread1");
                val.remove();
                print("after remove thread1");
            }
        });
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                val.set(1);
                for (int i = 0; i < 50; i++) {
                    int v = val.get();
                    v += 2;
                    val.set(v);
                }
                print("thread2");
                val.remove();
                print("after remove thread2");
            }
        });
        t1.start();
        t2.start();
        System.out.println("done");
    }
}

运行结果:

thread1:51
after remove thread1:null
done
thread2:101
after remove thread2:null

ThreadLocal的原理:

Thread中有一个threadLocals变量,它的类型是ThreadLocal.ThreadLocalMap,实际上是一个hashMap.
当使用set方法赋值时:

// ThreadLocal.java
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

getMap方法获得的是threadLocals,如果不为空,将当前threadLocal变量作为key,设置的值作为value放入进行存储,而如果为空,则利用createMap进行初始化.

// ThreadLocal.java
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

需要注意的是createMap中传入的是当前线程,因为需要为其设置threadLocals变量的值.而threadLocals中的key实际上是ThreadLocal变量,也就是可以理解为:thread1.threadLocals -> {ThreadLocal@xxx:1, ThreadLocal@yyy:"hello"}

但是ThreadLocal不支持继承,如果需要继承父线程的值,需要使用InheritableThreadLocal:

public class InheritThreadLocalTest {
    static InheritableThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>();

    static void print(String str) {
        System.out.println(str + ":" + inheritableThreadLocal.get());
    }

    public static void main(String[] args) {
        inheritableThreadLocal.set("hello");
        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                String initVal = inheritableThreadLocal.get();
                if (initVal != null) {
                    inheritableThreadLocal.set(initVal+" world");
                }
                print("thread 1");
            }
        });
        t.start();
        print("main");
        System.out.println("done");
    }
}

运行结果:

main:hello
done
thread 1:hello world

InheritableThreadLocal和ThreadLocal的set方法是一样的,区别在于createMap,它将变量写入了inheritableThreadLocals里:

    void createMap(Thread t, T firstValue) {
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }

在初始化线程的时候,init方法会设置inheritableThreadLocals:

    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals){
...
        if (inheritThreadLocals && parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
...
}

它会将父线程的inheritableThreadLocals都复制到自己的inheritableThreadLocals中.

最后编辑于：2021.09.10 15:43:38

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并发编程之美读书笔记 1

线程的概念

如何创建线程

wait与notify

wait

notify

生产者消费者

join

sleep

yield

中断

上下文切换与死锁

守护线程与用户线程

ThreadLocal

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