如何使用Lock ？Lock 與synchronized 區別是什麼？

Lock 

 

就像同步塊一樣，java.util.concurrent.locks.Lock是線程同步機制。但是，鎖比同步塊更靈活，更復雜。由於Lock是接口，因此需要使用其實現之一在應用程序中使用Lock。 ReentrantLock是Lock接口的一種此類實現。

代碼實例

Lock lock = new ReentrantLock();
 
lock.lock();
 

 
lock.unlock();

Lock 與 synchronized 不同之處

1. Lock可以設置超時.而synchronized 則不可以設置。

Lock.tryLock(long timeout, TimeUnit timeUnit)

2.synchronized 必須是在同一方法中。lock() 和unlock()可以在不同的方法中。 

Lock的使用實例

在此示例中，程序將模擬打印的行爲。您可以在不同的時間間隔內或同時向打印任務提交多個打印作業。打印機將從打印隊列中取出一份作業並進行打印。其餘工作將在那裏等待。一旦打印機完成了手頭的打印作業，它將從隊列中選擇另一個作業並開始打印。保持循環進行。

public class PrinterQueue {
    private final Lock queueLock = new ReentrantLock();

    public void printJob(Object document) {
        queueLock.lock();
        try {
            long duration = new Random().nextInt(10000);
            System.out.println(Thread.currentThread().getName() + ": PrintQueue: Printing a Job during "
                               + (duration / 1000) + " seconds :: Time - " + new Date());
            Thread.sleep(duration);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            queueLock.unlock();
            System.out.printf("%s: The document has been printed\n", Thread.currentThread().getName());

        }
    }
}

class PrintingJob implements Runnable {
    private PrinterQueue printerQueue;

    public PrintingJob(PrinterQueue printerQueue) {
        this.printerQueue = printerQueue;
    }

    @Override
    public void run() {
        System.out.printf("%s: Going to print a document\n", Thread.currentThread().getName());
        printerQueue.printJob(new Object());
    }
}

測試

public class PrinterQueue {
    private final Lock queueLock = new ReentrantLock();

    public void printJob(Object document) {
        queueLock.lock();
        try {
            long duration = new Random().nextInt(10000);
            System.out.println(Thread.currentThread().getName() + ": PrintQueue: Printing a Job during "
                               + (duration / 1000) + " seconds :: Time - " + new Date());
            Thread.sleep(duration);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            queueLock.unlock();
            System.out.printf("%s: The document has been printed\n", Thread.currentThread().getName());

        }
    }
}

 效果

"D:\Program Files\Java\jdk1.8.0_221\bin\java.exe" -Dvisualvm.id=15276093588700 "-javaagent:D:\Program Files\JetBrains\Toolbox\apps\IDEA-U\ch-0\201.6668.121\lib\idea_rt.jar=53022:D:\Program Files\JetBrains\Toolbox\apps\IDEA-U\ch-0\201.6668.121\bin" -Dfile.encoding=UTF-8 -classpath "D:\Program Files\Java\jdk1.8.0_221\jre\lib\charsets.jar;D:\Program Files\Java\jdk1.8.0_221\jre\lib\deploy.jar;D:\Program Files\Java\jdk1.8.0_221\jre\lib\ext\access-bridge-64.jar;D:\Program Files\Java\jdk1.8.0_221\jre\lib\ext\cldrdata.jar;D:\Program Files\Java\jdk1.8.0_221\jre\lib\ext\dnsns.jar;D:\Program Files\Java\jdk1.8.0_221\jre\lib\ext\jaccess.jar;D:\Program Files\Java\jdk1.8.0_221\jre\lib\ext\jfxrt.jar;D:\Program Files\Java\jdk1.8.0_221\jre\lib\ext\localedata.jar;D:\Program Files\Java\jdk1.8.0_221\jre\lib\ext\nashorn.jar;D:\Program Files\Java\jdk1.8.0_221\jre\lib\ext\sunec.jar;D:\Program Files\Java\jdk1.8.0_221\jre\lib\ext\sunjce_provider.jar;D:\Program 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com.demo.lock.LockExample
Thread 1: Going to print a document
Thread 9: Going to print a document
Thread 8: Going to print a document
Thread 5: Going to print a document
Thread 4: Going to print a document
Thread 6: Going to print a document
Thread 7: Going to print a document
Thread 3: Going to print a document
Thread 2: Going to print a document
Thread 0: Going to print a document
Thread 1: PrintQueue: Printing a Job during 2 seconds :: Time - Tue May 26 12:48:07 CST 2020
Thread 1: The document has been printed
Thread 9: PrintQueue: Printing a Job during 6 seconds :: Time - Tue May 26 12:48:09 CST 2020
Thread 9: The document has been printed
Thread 8: PrintQueue: Printing a Job during 2 seconds :: Time - Tue May 26 12:48:15 CST 2020
Thread 8: The document has been printed
Thread 5: PrintQueue: Printing a Job during 1 seconds :: Time - Tue May 26 12:48:18 CST 2020
Thread 5: The document has been printed
Thread 4: PrintQueue: Printing a Job during 9 seconds :: Time - Tue May 26 12:48:20 CST 2020
Thread 4: The document has been printed
Thread 6: PrintQueue: Printing a Job during 9 seconds :: Time - Tue May 26 12:48:29 CST 2020
Thread 6: The document has been printed
Thread 7: PrintQueue: Printing a Job during 7 seconds :: Time - Tue May 26 12:48:39 CST 2020
Thread 7: The document has been printed
Thread 3: PrintQueue: Printing a Job during 0 seconds :: Time - Tue May 26 12:48:47 CST 2020
Thread 3: The document has been printed
Thread 2: PrintQueue: Printing a Job during 2 seconds :: Time - Tue May 26 12:48:47 CST 2020
Thread 2: The document has been printed
Thread 0: PrintQueue: Printing a Job during 8 seconds :: Time - Tue May 26 12:48:50 CST 2020
Thread 0: The document has been printed

Process finished with exit code 0

 該示例的主要的在PrinterQueue類的printJob（）方法中。當我們想使用鎖來實現關鍵部分並確保只有一個執行線程運行代碼塊時，我們必須創建一個ReentrantLock對象。在方法的開始，我們必須使用lock（）方法來控制鎖。

在關鍵部分的末尾，我們必須使用unlock（）方法釋放鎖的控制權，並允許其他線程運行該關鍵部分。如果您在關鍵部分的末尾不調用unlock（）方法，則正在等待該塊的其他線程將永遠處於等待狀態，從而導致死鎖情況。如果您在關鍵部分中使用try-catch塊，請不要忘記將包含unlock（）方法的句子放在finally部分中