java - 通知和等待对于同步块(synchronized block)不起作用

Question

这段代码是对著名的 8 皇后拼图的不同实现尝试。我尝试为此使用多线程。以下代码段是迄今为止的实现。但有一个问题，wait方法永远等待主线程。我添加了一些 SOut 来简化测试，以便确认它被卡住了。

主类:

public class MainClass {

    public static void main(String[]args)
    {
        Queen.board[1][3]=true;
        Queen queen=new Queen();
        queen.placeNextQueen();
    }
}

女王级

public class Queen {

    private static final Object syncOb=new Object();
    public static boolean[][]board=new boolean[10][10];
    public static int onBoard=0;

    private int callbacks=1;

    Thread runRow;
    Thread runCol;
    Thread runLDiag;
    Thread runRDiag;

    boolean rowSafe=true;
    boolean colSafe=true;
    boolean rDiagSafe=true;
    boolean lDiagSafe=true;

    public Queen()
    {
    }

    public void placeNextQueen()
    {
        final Queen queen=this;
        if(++onBoard<8)
        {
            for(int i=0;i<7;i++)
            {
                System.out.println("*******");
                callbacks=1;
                for(int r=0;r<7;r++)
                {
                    final int finalI = i;
                    final int finalR = r;

                    runRow=new Thread() {
                        @Override
                        public void run() {
                            isRowSafe(queen,finalI);
                        }
                    };

                    runCol=new Thread() {
                        @Override
                        public void run() {
                            isColSafe(queen,finalR);
                        }
                    };
                    runRDiag=new Thread() {
                        @Override
                        public void run() {
                            isRDiagSafe(queen,finalI,finalR);
                        }
                    };
                    runLDiag=new Thread() {
                        @Override
                        public void run() {
                            isLDiagSafe(queen,finalI,finalR);
                        }
                    };

                    try
                    {
                        runRow.run();
                        runCol.run();
                        runRDiag.run();
                        runLDiag.run();
                        synchronized(syncOb) {

                            syncOb.wait();
                            System.out.println("WAIT OVER*****************");
                        }
                        if(rowSafe && colSafe && rDiagSafe && lDiagSafe)
                        {
                            board[i][r]=true;

                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
                System.out.println("INNER LOOP OVER*****************");
            }

            System.out.println("TO SHOW BOARD*****************");
            showBoard();

        }
    }

    public void showBoard() {

        System.out.println("SHOW BOARD*****************");
        for(int i=0;i<8;i++)
        {
            System.out.print("|");
            for(int r=0;r<8;r++)
            {
                if(board[i][r])
                {
                    System.out.print("*");
                }
                else
                    System.out.print(" ");
                System.out.print("|");
            }
            System.out.println();
        }
    }

    public void callBack()
    {

        System.out.println("CALLBACK*****************"+rowSafe+" "+colSafe+" "+rDiagSafe+" "+lDiagSafe+" "+callbacks);
        if(callbacks++ ==4||(!rowSafe && !colSafe && !rDiagSafe && !lDiagSafe))
        {
            runRow.interrupt();
            runCol.interrupt();
            runRDiag.interrupt();
            runLDiag.interrupt();
            synchronized (syncOb) {
                System.out.println("NOTIFY*****************");
                syncOb.notifyAll();
                System.out.println("NOTIFYed*****************");

            }

        }
    }


    public void isRowSafe(Queen q,int row)
    {
        System.out.println("------------ SAFE");
        for(int i=0;i<7;i++)
        {
            System.out.println("----------- LOOP");
            if(board[row][i])
            {
                System.out.println("--------- IF");
                rowSafe= false;
            }
        }
        rowSafe= true;
        q.callBack();
    }

    public void isColSafe(Queen q,int col)
    {
        System.out.println("||||||||| SAFE");
        for(int i=0;i<7;i++)
        {
            System.out.println("||||||||| LOOP");
            if(board[i][col])
            {
                System.out.println("||||||||| IF");
                colSafe = false;
            }

        }
        colSafe= true;
        q.callBack();
    }

    public void isRDiagSafe(Queen q,int row, int col)
    {
        int initRow=row;
        int initCol=col;

        System.out.println("////////// SAFE");
        //up diagonal
        if(row!=0)
            for (int i=initRow-1;i>=0;i--)
            {
                System.out.println("///////// UP"+i+","+col);
                if(++col>7)
                {
                    rDiagSafe = true;
                    q.callBack();
                    return;
                }
                if(board[i][col])
                    rDiagSafe= false;
            }

        col=initCol;

        //down diagonal
        if(row!=7)
            for(int i=initRow+1;i<8;i++)
            {
                System.out.println("/////////// DOWN"+i+","+col);
                if(--col<0) {
                    rDiagSafe = true;
                    q.callBack();
                    return;
                }
                if(board[i][col])
                    rDiagSafe= false;
            }

        q.callBack();
    }

    public void isLDiagSafe(Queen q,int row, int col)
    {
        System.out.println("DDDDDDDDDDDDDDD SAFE");
        int initRow=row;
        int initCol=col;

        //up diagonal
        if(row!=0)
            for (int i=initRow-1;i>=0;i--)
            {
                System.out.println("DDDDDDDDDDDDDDD UP");
                if(--col>7) {
                    lDiagSafe = true;
                    q.callBack();
                    return;
                }
                if(board[i][col])
                    lDiagSafe= false;
            }

        col=initCol;

        //down diagonal
        if(row!=7)
            for(int i=initRow+1;i<8;i++)
            {
                System.out.println("DDDDDDDDDDDDDDD DOWN");
                if(++col<0) {
                    lDiagSafe = true;
                    q.callBack();
                    return;
                }
                if(board[i][col])
                    lDiagSafe= false;
            }

        q.callBack();
    }

}

我看不出这里有什么问题，但线程没有唤醒。请有人帮我找出错误。

Answer 1

代码存在几个问题。一是调用“wait()”的线程不是进行数据更改或读取数据的线程。与数据交互的线程完全不同步，并且没有使用锁对象。没有人调用“notify()”。

https://docs.oracle.com/javase/specs/jls/se8/html/jls-17.html#jls-17.2 https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait--

“导致当前线程等待，直到另一个线程调用该对象的notify()方法或notifyAll()方法。”

为什么需要“wait()”而不是其他形式的同步？