我的Java代碼未同步

Question

class MultiplyTable1 {
    synchronized void printTable(int n) {
        for (int i = 1; i <= 5; i++) {
            System.out.println(n * i);
            try {
                Thread.sleep(500);
            } catch (Exception e) {
                System.out.println(e);
            }
        }
    }
}

class ThreadOne1 extends Thread {
    MultiplyTable1 mtObj = new MultiplyTable1();

    public void run() {
        mtObj.printTable(2);
    }
}

class ThreadTwo2 extends Thread {
    MultiplyTable1 mtObj = new MultiplyTable1();

    public void run() {
        mtObj.printTable(100);
    }
}

public class ThreadDemoDupe {

    public static void main(String[] args) {
        ThreadOne1 t1 = new ThreadOne1();
        ThreadTwo2 t2 = new ThreadTwo2();
        t1.start();
        t2.start();
    }
}

OUTPUT：

100
2
200
4
300
6
8
400
10
500

我的代碼將如何獲得如下輸出：

2
4
6
8
10
100
200
300
400
500

要么

100
200
300
400
500
2
4
6
8
10

我沒有更多細節可提供。

Answer 1

您正在創建兩個單獨的MultiplyTable1對象。 synchronized實例方法有效地使用：

synchronized (this) {
    ...
}

因此，如果您在兩個不同的對象上調用該方法，它們仍然可以並行運行。 若要查看同步效果，您需要在一個公共對象上進行同步。 您可以通過將其更改為不在MultiplyTable1對象本身上進行同步來實現，也可以僅將兩個線程賦予相同的MultiplyTable1對象。 例如：

class MultiplyTable1 {
    synchronized void printTable(int n) {
        for (int i = 1; i <= 5; i++) {
            System.out.println(n*i);
            try {
                Thread.sleep(500);
            }
            catch (InterruptedException e) {
                System.out.println(e);
            }
        }
    }
}

// Prefer implementing Runnable over extending Thread.
// In reality I'd only have a single class and parameterize
// the value passed to the printTable method, but...
class Runnable1 implements Runnable {
    private final MultiplyTable1 table;

    Runnable1(MultiplyTable1 table) {
        this.table = table;
    }

    @Override public void run() {
        table.printTable(2);
    }
}

class Runnable2 implements Runnable {
    private final MultiplyTable1 table;

    Runnable2(MultiplyTable1 table) {
        this.table = table;
    }

    @Override public void run() {
        table.printTable(100);
    }
}

public class ThreadDemoDupe {
    public static void main(String[] args) {
        MultiplyTable1 table = new MultiplyTable1();
        Thread t1 = new Thread(new Runnable1(table));
        Thread t2 = new Thread(new Runnable2(table));
        t1.start();
        t2.start();
    }
}