private synchronized void function() {}

private void function() {
    synchronized (object) {}
}

鎖加在 object 物件上面。該物件可以是當前物件（object == this），也可以是當前類的Class物件（object == MyClass.class）

簡單例子，多執行緒操作同一賬戶

執行緒類：

public class SynchronizedThread extends Thread {

	private Account account;
	private int cash;

	public SynchronizedThread(String name, Account account, int cash) {
		super(name);
		this.account = account;
		this.cash = cash;
	}

	@Override
	public void run() {
		account.plus(cash);
	}

	public static void main(String[] args) {
		Account at = new Account(100);
		for (int i = 0; i < 5; i++) {
			new SynchronizedThread("A" + i, at, -10).start();
		}
	}

}
 

賬戶類(未同步)：

public class Account {

	private int money;

	public Account(int money) {
		this.money = money;
	}

	// 業務方法
	public void plus(int cash) {
		try {
			int last = money;
			Thread.sleep(1); // 模擬一個延遲
			money += cash;
			System.out.println(Thread.currentThread().getName() + ": 錢包: " + last + " 消費: " + cash + " 餘額: " + money);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

}
 

未同步時，執行結果：

進行執行緒同步後：

同步方法：

public synchronized void plus(int cash) {
		try {
			int last = money;
			Thread.sleep(1); // 模擬一個延遲
			money += cash;
			System.out.println(Thread.currentThread().getName() + ": 錢包: " + last + " 消費: " + cash + " 餘額: " + money);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

public void plus(int cash) {
		try {
			synchronized (this) {
				int last = money;
				Thread.sleep(1); // 模擬一個延遲
				money += cash;
				System.out.println(Thread.currentThread().getName() + ": 錢包: " + last + " 消費: " + cash + " 餘額: " + money);
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}