Java 线程程序无法使用 wait() 和 notifyAll()

Question

下面是我的程序。总是线程 0 获取打印机，其他线程不获取它。有一个打印机对象，我希望多个作业线程使用打印机。如何使这个程序工作，以便所有作业都得到打印机。对我来说，代码流似乎没问题。正在同步单个打印机对象。请帮忙。

    package classesTesting;

    public class PrinterQueue {

        final static Printer printer = new Printer();;

        public static void main(String[] args) {
            // TODO Auto-generated method stub
            System.out.println("In Main");

            for (int i = 0; i < 5; i++) {
                new Thread(new Jobs(), "Thread - " + i).start();
                System.out.println("started " + i + " thread");
            }

        }

    }

    class Printer {
        private boolean isUsed;

        Printer() {
            this.isUsed = false;
        }

        public void setUsed(boolean used) {
            this.isUsed = used;
        }

        public boolean isUsed() {

            return this.isUsed;
        }
    }

    class Jobs implements Runnable {

        String name;
        boolean isDataAvailble;

        Jobs() {        
            this.isDataAvailble = true;
        }

        public void setNoData(boolean noData) {
            this.isDataAvailble = false;
        }

        @Override
        public void run() {

            while (isDataAvailble) {

                if (PrinterQueue.printer.isUsed()) {
                    try {
                        System.out.println(Thread.currentThread()
                                + "WAITING FOR PRINTER");
                        synchronized (PrinterQueue.printer) {
                            PrinterQueue.printer.wait();
                        }
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                } else {
                    synchronized (PrinterQueue.printer) {
                        System.out.println(Thread.currentThread() + "GOT PRINTER");
                        PrinterQueue.printer.setUsed(true);
                        try {
                            Thread.sleep(3000);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        PrinterQueue.printer.setUsed(false);
                        PrinterQueue.printer.notify();
                    }
                }
            }

            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

    }

您好，我修改了我的程序，先获取锁再检查条件。即使这样，线程 0 也总是得到打印机。其他线程饿死。

修改后的程序:

    package classesTesting;

    public class PrinterQueue {

        static Printer printer;

        public static void main(String[] args) {
            // TODO Auto-generated method stub
            System.out.println("In Main");

            printer = new Printer();

            for (int i = 0; i < 5; i++) {
                Jobs j1 = new Jobs();
                j1.setPrinter(printer);

                Thread t1 = new Thread(j1, "Thread - " + i);
                t1.start();

                System.out.println("started " + i + " thread");
            }

        }

    }

    class Printer {
        private boolean isUsed;

        Printer() {
            this.isUsed = false;
        }

        public void setUsed(boolean used) {
            this.isUsed = used;
        }

        public boolean isUsed() {

            return this.isUsed;
        }
    }

    class Jobs implements Runnable {

        String name;
        Printer printer;

        public Printer getPrinter() {
            return printer;
        }

        public void setPrinter(Printer printer) {
            this.printer = printer;
        }

        boolean isDataAvailble;

        Jobs() {
            this.isDataAvailble = true;
        }

        public void setNoData(boolean noData) {
            this.isDataAvailble = false;
        }

        @Override
        public void run() {

            while (isDataAvailble) {
                synchronized (PrinterQueue.printer) {
                    if (this.printer.isUsed()) {
                        try {
                            System.out.println(Thread.currentThread()
                                    + "WAITING FOR PRINTER");

                            PrinterQueue.printer.wait();

                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }

                    else {

                        System.out.println(Thread.currentThread() + "GOT PRINTER");

                        PrinterQueue.printer.setUsed(true);

                        try {
                            Thread.sleep(3000);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }

                        PrinterQueue.printer.setUsed(false);
                        PrinterQueue.printer.notify();
                    }
                }
            }

        }

    }

Answer 1

如果您希望资源以公平的方式供所有线程使用，最好使用带有fair = true 参数的ReentrantLock。也永远不要依赖以并发方式更改的非 volatile 变量。这是固定代码:

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class PrinterQueue {
    static Printer printer;

    public static void main(String[] args) {
        System.out.println("In Main");
        printer = new Printer();
        for (int i = 0; i < 5; i++) {
            // I added printer constructor parameter to pass the same printer
            // to all the Jobs
            new Thread(new Jobs(printer), "Thread - " + i).start();
            System.out.println("started " + i + " thread");
        }
    }
}

class Printer {
    // internally printer holds a fair ReentrantLock
    Lock lock = new ReentrantLock(true);

    // call this to get the printer
    public void acquire() {
        lock.lock();
    }

    // call this to release the printer, so it's available for other threads
    public void release() {
        lock.unlock();
    }
}

class Jobs implements Runnable {
    // Declare isDataAvailble as volatile as you're going to change it from another thread
    volatile boolean isDataAvailble;
    private final Printer printer;

    // constructor now takes the printer argument
    Jobs(Printer printer) {
        this.isDataAvailble = true;
        this.printer = printer;
    }

    @Override
    public void run() {
        try {
            while (isDataAvailble) {
                System.out.println(Thread.currentThread()
                        + "Trying to get the printer");
                // get the printer
                this.printer.acquire();
                try {
                    System.out.println(Thread.currentThread()
                            + "Printer acquired!");
                    // use it
                    Thread.sleep(3000);
                } finally {
                    // Release the printer. Better to do it in finally block
                    // so you will release it even if some unexpected exception occurs
                    this.printer.release();
                }
            }

            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}