java - 多级锁会导致多线程程序死锁吗？

Question

我有一个包含编写者和读者的程序，他们的访问权限由监视器控制。

所以，这应该会挨饿，但我陷入了僵局。我想知道为什么，然后我记得我又放了一个锁，我认为在读取器内部的读取方法中没有必要这样做，以保护我的全局变量免受不一致的影响。我认为这不会导致任何死锁，因为我可以运行线程 10000 次而不会发生任何死锁，但是当我必须进行实验室演示时，我认为它在第 10010 个线程处死锁。我不明白为什么它会这样做。另外，我没想到它会挨饿，但显然它应该挨饿。

我的问题是:那些多级锁是否造成了死锁？如果不是，是什么原因造成的？!

    import java.io.*;
    import java.io.IOException;
    import java.util.*;

    public class Writer extends Thread{

    private int number;

    public Writer(int number)
    {
        this.number = number;
    }

    public int getNumber()
    {
        return number;
    }

        public static void Write(String filename){

        try {

            String content = RandomString();


            File f = new File(filename);

            if (!f.exists())
            {
                f.createNewFile();
            }


            PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("Task1out.txt", true)));
            out.println(content);
            out.close();


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

    public static String RandomString(){

        String chars = new String("0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ");
        int n = chars.length();

        String randomString = new String();
        Random r = new Random();

            for (int i=0; i<100; i++)
            {
                randomString = randomString + chars.charAt(r.nextInt(n));
            }

        System.out.println("RandomString() generated: " + randomString);

        return randomString;

    }



    public void run(){

        try{

        //FileControl fc = new FileControl();

            int number = this.getNumber();


            for(int i = 0; i <1000; i++) //CHANGE IT TO 1000
            {
                main.fc.WriterEntry(number);

                //write file random characters (must append)

                Write("Task1out.txt");

                main.fc.WriterExit(number);

            }
        } catch(InterruptedException e)
        {
            System.out.println("Interrupted Exception caught");
        }

    }


}

这是作家类。

    import java.io.BufferedWriter;
    import java.io.BufferedReader;
    import java.io.File;
    import java.io.FileWriter;
    import java.io.FileReader;
    import java.io.IOException;
    import java.util.*;
    import java.util.concurrent.locks.Condition;
    import java.util.concurrent.locks.Lock;
    import java.util.concurrent.locks.ReentrantLock;



public class Reader extends Thread{


    private int number;

    public Reader(int number)
    {
        this.number = number;
    }


    public int getNumber()
    {
        return number;
    }

        public static synchronized void Read(String filename)throws InterruptedException{

        BufferedReader br = null;





            main.lock.lock(); //lock
        try{




        try {


            String line;
            char[] chars = new char[100];
            int readIndex2 = 0;
            int addToIndex = 0;



            br = new BufferedReader(new FileReader(filename));


            int initialReadIndex = main.getIndex();




            System.out.println("initial read index: " + initialReadIndex);

            while ((line = br.readLine()) != null && readIndex2 < initialReadIndex+100 && addToIndex < 100) {

                for(int i = 0; i< 100; i++)
                {
                    if (initialReadIndex == readIndex2 || initialReadIndex < readIndex2)
                    {

                        if(line.length() > addToIndex)
                        {




                        chars[i] = line.charAt(i);
                        addToIndex++;
                        }


                    }
                    else
                    {
                        readIndex2++;
                    }
                }
                System.out.println(chars);
            }

            if(line == null)
            {
                System.out.println("nothing to read");
            }



            main.incrementIndex(addToIndex);


            System.out.println("current read index: " + (initialReadIndex + addToIndex));





        } catch (IOException e) {
            e.printStackTrace();
            System.out.println("buffered reader exception");
        } finally {


            try {


                if (br != null)
                    {

                    br.close();
                    }
            } catch (IOException ex) {
                ex.printStackTrace();
                System.out.println("exception during closing");
            }
        }
        }finally{
            main.lock.unlock(); //lock

        }

        }


    public void run(){

        try{


        //FileControl fc = new FileControl();


        int number = this.getNumber();


            for(int i = 0; i <1000; i++) //CHANGE IT TO 1000
            {
                main.fc.ReaderEntry(number);

                //read file

                Read("Task1out.txt");

                main.fc.ReaderExit(number);
            }
        } catch(InterruptedException e)
        {
            System.out.println("Interrupted Exception caught");
        }

    }



        }

这是读者类。

 import java.io.BufferedWriter;
    import java.io.BufferedReader;
    import java.io.File;
    import java.io.FileWriter;
    import java.io.FileReader;
    import java.io.IOException;
    import java.util.concurrent.locks.Condition;
    import java.util.concurrent.locks.Lock;
    import java.util.concurrent.locks.ReentrantLock;

    public class main{

    public static FileControl fc = new FileControl();

    final static Lock lock = new ReentrantLock();

    public static int readIndex;

    public static void incrementIndex(int increment) {


                readIndex = readIndex + increment;

    }

    public static int getIndex()
    {
        return readIndex;
    }



    public static void main(String[] args) throws InterruptedException {



            Writer [] writer = new Writer[10];
            Reader [] reader = new Reader[10];

            for(int i = 0; i < 10; i++)
            {
                reader[i] = new Reader(i);
                writer[i] = new Writer(i);
                //creating readers and writers

            }

            for(int i = 0; i < 10; i++)
            {
                //anonymous threads
                //(new Thread(new Writer())).start();
                //(new Thread(new Reader())).start();

                reader[i].start();
                writer[i].start();

            }




            for(int i = 0; i < 10; i++)
            {
                try{
                    reader[i].join();
                    writer[i].join();
                } catch(InterruptedException e){
                    e.printStackTrace();
                }


            }






        }

}

这是主类。

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


    public class FileControl {
    final Lock lock = new ReentrantLock();
    final Condition writers = lock.newCondition();
    final Condition readers = lock.newCondition();
    int activereaders = 0;
    int waitingwriters = 0;
    boolean writing = false;

    public void WriterEntry(int number)throws InterruptedException{
        lock.lock();
        try{
                if(writing == true || activereaders > 0){
                    waitingwriters++;
                    System.out.println("Writer thread " + number + " : waiting to write");
                    writers.await();
                    waitingwriters--;
                }
                System.out.println("Writer thread " + number + " : ready to write");

                writing = true;
           }
        finally{
            lock.unlock();
        }


    }



    public void WriterExit(int number)throws InterruptedException{
        lock.lock();
        try{
            System.out.println("Writer thread " + number + " : finished to write");

            System.out.println("writers " + waitingwriters + "readers " + activereaders); //test

            if(waitingwriters > 0)
                writers.signal();
            else{
                writing = false;
                readers.signal();
            }
        }
        finally{
            lock.unlock();
        }

    }


    public void ReaderEntry(int number)throws InterruptedException{
        lock.lock();
        try{

            if(writing == true || waitingwriters > 0){ //remove activereaders > 0
                System.out.println("Reader thread " + number + " : waiting to read");
                readers.await();
                activereaders++;
            }


            System.out.println("Reader thread " + number + " : ready to read");
        }
        finally{
            lock.unlock();
        }

    }

    public void ReaderExit(int number)throws InterruptedException{
        lock.lock();
        try{



        activereaders--;



        System.out.println("Reader thread " + number + " : finished to read");

        System.out.println("writers " + waitingwriters + "readers " + activereaders); //test

            if(activereaders == 0)
            {
                if(waitingwriters > 0)
                {
                    writers.signal();
                }
                else
                {
                    readers.signal();
                }
            }
        }
        finally{
            lock.unlock();
        }

    }


}

这是显示器。

pseudocode for the monitor

Answer 1

每当您有多个锁 A、B 和 C 时，如果您不保证代码尝试以相同的顺序获取所述锁，则可能会出现死锁。

final Lock A = new ReentrantLock();
final Lock B = new ReentrantLock();
final Lock C = new ReentrantLock();

A、B、C 或 C、B、A 或 A、C、B - 只要顺序一致即可。

当您尝试使用一个代码路径:A、B、C 时，就会出现问题另一个尝试 C、B、A。

正如你可能猜到的那样，由于 A 和 C 都被持有，两者之一将获得 B，然后两者都会陷入僵局。 (也就是说，资源锁定图中有一个循环)

从形式上讲，只有满足以下所有条件，才会出现死锁:

1. 无抢占:分配后系统不会释放资源；它们只能通过持有过程来释放。
2. 循环等待:如上所述。
3. 互斥:在任何给定时间只有一个进程可以使用资源。
4. 资源持有:进程当前至少持有一个资源，并正在请求/等待另一个进程持有的其他资源。

最好的解决方案是确保顺序一致或锁定在更高(单一)级别。另一种选择是使用锁定库，该库在尝试锁定时会超时(或使用条件并编写自己的包装器来执行此操作)。但这种方法不适合胆小的人。此方法的某些实现将等待随机时间并重试，但随着锁数量的增加，这可能会非常低效。

资源:

• 这是一篇关于分析 Java 死锁的实用文章，您可以阅读可能感兴趣: http://www.journaldev.com/1058/java-deadlock-example-and-how-to-analyze-deadlock-situation
• 您还可以使用 JCarder 等开源工具来查找死锁: http://www.jcarder.org/对于具有大转储的程序可以比尝试 grok 转储文件更容易。

附注我实际上并没有阅读你的太多代码，因为它的格式很差并且不是一个最小的示例(即对于我们的目的来说太冗长了)。但这个建议应该从理论角度回答你的问题。