Reference StackOverflow question:

http://stackoverflow.com/ques...

However, I still have more to say, so please give me your advice and discussion!

Added:

Run the following test program, the final result is not 20000, the results are overwritten many times in the middle.

public class Test {
    
    private static int staticVariableOne = 0;

    public Test() {
        super();
    }
    
    public static int getStaticVariableOne() {
        return staticVariableOne;
    }

    public static void setStaticVariableOne(int staticVariableOne) {
        Test.staticVariableOne = staticVariableOne;
    }
    
    private static void increaseStaticVariableOne() {
        staticVariableOne++;
    }
    
    public synchronized void add() {
        System.out.println("执行 add() 开始...");
        try {
            for (int i  = 0; i < 10000; i++) {
                increaseStaticVariableOne();
                Thread.sleep(1);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("执行 add() 完毕...");
    }
 
    public synchronized static void addStatic() {
        System.out.println("执行 addStatic() 开始...");
        try {
            for (int i  = 0; i < 10000; i++) {
                increaseStaticVariableOne();
                Thread.sleep(2);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("执行 addStatic() 完毕...");
    }

}

public class StudyMain {
    public static void main( String[] args ) {
        
        final Test insertData = new Test();
        Thread threadOne = new Thread() {
            @Override
            public void run() {
                insertData.add();
            }
        };
        
        Thread threadTwo = new Thread() {
            @Override
            public void run() {
                Test.addStatic();
            }
        };
        
        threadOne.start();
        threadTwo.start();
        try {
            threadOne.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        try {
            threadTwo.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("After 20000 times increasements, final staticVariableOne is "
        + Test.getStaticVariableOne());
    }
}