從JDK5.0開始，有兩種機制來保護程式碼塊不受到並行訪問的干擾。舊版本的Java使用synchronized關鍵字來達到這個目的，而JDK5.0引進了ReentrantLock()類。synchronize關鍵字自動提供一個鎖和相關的條件。
用ReentrantLock保護程式碼塊的結構如下：
Lock myLock = new ReentrantLock();

myLock.lock()
try{
    需要保護的程式碼塊;
    }
finally{
    myLock.unlock
    }

下面是用法示例：

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

class WorkThread extends Thread{
    private
 
 CourruptedData data = null ;
    private int type = 0 ;
    public WorkThread(CourruptedData _data,int _type){
        data = _data ;
        type = _type ;//快程序和滿程序的標記。
        super.start();
    }
    public void run(){
        data.performWork(type);
    }
}

public class CourruptedData {
    protected
 
 static int display = 1 ;
    protected static int change = 2 ;
    private WorkThread slowWorker = null ;
    private WorkThread fastWorker = null ;
    private int number = 0 ;
    private volatile double f = 1.1;

    Lock lock = new ReentrantLock();

    public CourruptedData() {
        number = 1
 
 ;
        slowWorker = new WorkThread(this,display);//-----------慢程序先啟動;
        fastWorker = new WorkThread(this,change);
    }

    /*
     * 關鍵字為synchronized時，方法修飾為同步方法，
     * 慢執行緒呼叫performWokr()時，休眠兩秒；
     * 此時，不允許第二個執行緒打斷第一個執行緒；
     * 等第一個執行緒執行完畢之後，第二個執行緒才能將number改為-1；
     */

    public void performWork(int type){
        if (type == display)
        {
            lock.lock();
            try{
                System.out.println("Number before sleeping:" + number);
                try{
                    slowWorker.sleep(2000);
                }catch (InterruptedException ie){
                    System.out.println("Error : "+ie);
                }
                System.out.println("Number after working up:"+ number);
            }
            finally{
                lock.unlock();
            }

        }
        if (type == change)
        {
            lock.lock();
            try{
                System.out.println("Change the numbering:");
                number = -1 ;
                System.out.println(number);
            }
            finally{
                lock.unlock();
            }

        }

    }


    public static void main(String args[])
    {
        CourruptedData test = new CourruptedData();
    }
}