ReentrantLock

灰太狼 2021-12-18 22:15 201阅读 0赞

在这篇文章中，我们将会讨论`Java`的`ReentrantLock`，它在某些方面和`synchronized`很相似，但是`ReentrantLock`更加灵活。我鼓励所有读者使用`ReentrantLock`，  
因为它本质上是外在的，开发人员可以更多地控制它，与`synchronized`不同。

下面是对于使用`synchronized`和`ReentrantLock`的对比：

*  在`synchronized`中，线程都是尝试去锁住共享对象的`monitor`，这里是不存在任何方案打断其它正在共享对象池中等待的线程；
 *  `ReentrantLock`本身就是一个对象，线程是去尝试锁住`ReentrantLock`类的实例，在这里存在方式可以打断其它在锁池中等待的线程。
 *  如果线程之间都在互相等待锁的释放，那么就会造成死锁的出现。比如，线程A等待线程B释放锁，线程B等待线程A释放锁。阻塞线程可能会饿死。当线程处于挂起状态时，可能会有另一个线程捕获对象监视
 *  通过重入，我们可以通过创建非阻塞线程来消除死锁的可能性。但是在重入锁定中我们可能会遇到活锁问题，非阻塞线程尝试一次又一次地捕获锁定但不能，因为它由另一个线程持有，它试图锁定由前一个线程捕获的另一个重入锁定。  
    在重入中，有一个公平的策略，更长的等待线程可以捕获锁定实例

`ReentrantLock`的一些重要方法：\`

lock():当调用该方法的时候，线程会锁定`Lock`的实例，如果`Lock`已经被锁住，线程就会等待`lock`的解锁；
    tryLock():在调用此方法时，如果它没有被锁定，Thread会锁定实例。如果Lock被锁定，则线程返回，线程不会等待，解除阻塞性质。
    getHoldCount():它返回给定线程的锁定保持数
    lockInterruptibly():如果锁对象没有被锁并且线程没有被其它的线程所中断，通过调用该方法，线程可以锁定实例。
    tryLock(long timeout,TimeUnit timeUnit):调用该方法的时候，如果实例没有被锁定，那么线程会锁定实例。如果`Lock`已经被锁，那么线程会在指定的时间间隔返回。
    unlock():释放`Lock`的实例

`ReentrantLock`工作原理：一旦一个线程被锁住，锁的持有者数量加1，当相同的线程释放掉该锁的时候，锁对象的持有数量减1.

例子：

package com.example.demo.thread;
    
    import java.util.concurrent.locks.ReentrantLock;
    
    public class Account {
        private ReentrantLock implicitLock= new ReentrantLock();
        private String name;
        private Integer balance=10000;
        public ReentrantLock getImplicitLock() {
            return implicitLock;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        public Integer getBalance() {
            return balance;
        }
        public void setBalance(Integer blance) {
            this.balance = blance;
        }
        public boolean debit(Integer amount){
            if(amount > balance){
                System.out.println(Thread.currentThread().getName() + " :: " +name + " says ::"+ amount + " grater than current balance" );
                return false;
            }
            balance = balance -amount;
            System.out.println(Thread.currentThread().getName() + " :: " + name + " says ::"+ amount + " Debited Success Fully" );
            return true;
        }
        public void credit(Integer amount){
            balance = balance +amount;
            System.out.println(Thread.currentThread().getName() + " :: " + name + " says ::"+ amount + " Credited Success Fully" );
        }
    }
    
    
    package com.example.demo.thread;
    
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    import java.util.concurrent.locks.ReentrantLock;
    
    public class AcountTransfer {
        private ReentrantLock lock= new ReentrantLock();
        public void transfer(Account from,Account to,Integer amount){
            boolean transfer = false;
            try{
                if(lock.tryLock()){
                    System.out.println(Thread.currentThread().getName() + " says accuire lock");
                    boolean flag = from.debit(amount);
                    if(flag){
                        to.credit(amount);
                    }
                    System.out.println(Thread.currentThread().getName()+ " :: " + from.getName() + " says :: now balance is " + from.getBalance());
                    System.out.println(Thread.currentThread().getName()+ " :: " + to.getName() + " says :: now balance is " + to.getBalance());
                    transfer = true;
                }else{
                    System.out.println(Thread.currentThread().getName() + " says fail to accuire both lock Try again");
                    //try again
                    transfer(from,to,amount);
                }
            }catch(Exception ex){
                ex.printStackTrace();
            }finally{
                if(transfer){
                    lock.unlock();
                }
            }
        }
        public static void main(String[] args) {
            ExecutorService service = Executors.newFixedThreadPool(3);
            Account from = new Account();
            from.setBalance(20000);
            from.setName("Shamik Mitra");
            Account to = new Account();
            to.setName("Samir Mitra");
            AcountTransfer transfer = new AcountTransfer();
            Runnable a = () -> {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                transfer.transfer(from, to, 200);
                System.out.println(Thread.currentThread().getName() +" says :: Transfer successfull");
            };
            Runnable b = () -> {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                transfer.transfer(to, from, 1000);
                System.out.println(Thread.currentThread().getName() +" says :: Transfer successfull");
            };
            for(int i=0;i<4;i++){
                service.submit(a);
                service.submit(b);
            }
        }
    }

运行结果：

D:\Java8\jdk1.8.0_211\bin\java.exe -Dvisualvm.id=280311746376900 "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2019.1.1\lib\idea_rt.jar=55497:C:\Program Files\JetBrains\IntelliJ IDEA 2019.1.1\bin" -Dfile.encoding=UTF-8 -classpath D:\Java8\jdk1.8.0_211\jre\lib\charsets.jar;D:\Java8\jdk1.8.0_211\jre\lib\deploy.jar;D:\Java8\jdk1.8.0_211\jre\lib\ext\access-bridge-64.jar;D:\Java8\jdk1.8.0_211\jre\lib\ext\cldrdata.jar;D:\Java8\jdk1.8.0_211\jre\lib\ext\dnsns.jar;D:\Java8\jdk1.8.0_211\jre\lib\ext\jaccess.jar;D:\Java8\jdk1.8.0_211\jre\lib\ext\jfxrt.jar;D:\Java8\jdk1.8.0_211\jre\lib\ext\localedata.jar;D:\Java8\jdk1.8.0_211\jre\lib\ext\nashorn.jar;D:\Java8\jdk1.8.0_211\jre\lib\ext\sunec.jar;D:\Java8\jdk1.8.0_211\jre\lib\ext\sunjce_provider.jar;D:\Java8\jdk1.8.0_211\jre\lib\ext\sunmscapi.jar;D:\Java8\jdk1.8.0_211\jre\lib\ext\sunpkcs11.jar;D:\Java8\jdk1.8.0_211\jre\lib\ext\zipfs.jar;D:\Java8\jdk1.8.0_211\jre\lib\javaws.jar;D:\Java8\jdk1.8.0_211\jre\lib\jce.jar;D:\Java8\jdk1.8.0_211\jre\lib\jfr.jar;D:\Java8\jdk1.8.0_211\jre\lib\jfxswt.jar;D:\Java8\jdk1.8.0_211\jre\lib\jsse.jar;D:\Java8\jdk1.8.0_211\jre\lib\management-agent.jar;D:\Java8\jdk1.8.0_211\jre\lib\plugin.jar;D:\Java8\jdk1.8.0_211\jre\lib\resources.jar;D:\Java8\jdk1.8.0_211\jre\lib\rt.jar;D:\demo\target\classes;C:\maven\mvn_repo3.0\org\springframework\boot\spring-boot-starter-web\2.1.5.RELEASE\spring-boot-starter-web-2.1.5.RELEASE.jar;C:\maven\mvn_repo3.0\org\springframework\boot\spring-boot-starter\2.1.5.RELEASE\spring-boot-starter-2.1.5.RELEASE.jar;C:\maven\mvn_repo3.0\org\springframework\boot\spring-boot-starter-logging\2.1.5.RELEASE\spring-boot-starter-logging-2.1.5.RELEASE.jar;C:\maven\mvn_repo3.0\ch\qos\logback\logback-classic\1.2.3\logback-classic-1.2.3.jar;C:\maven\mvn_repo3.0\ch\qos\logback\logback-core\1.2.3\logback-core-1.2.3.jar;C:\maven\mvn_repo3.0\org\apache\logging\log4j\log4j-to-slf4j\2.11.2\log4j-to-slf4j-2.11.2.jar;C:\maven\mvn_repo3.0\org\apache\logging\log4j\log4j-api\2.11.2\log4j-api-2.11.2.jar;C:\maven\mvn_repo3.0\org\slf4j\jul-to-slf4j\1.7.26\jul-to-slf4j-1.7.26.jar;C:\maven\mvn_repo3.0\javax\annotation\javax.annotation-api\1.3.2\javax.annotation-api-1.3.2.jar;C:\maven\mvn_repo3.0\org\yaml\snakeyaml\1.23\snakeyaml-1.23.jar;C:\maven\mvn_repo3.0\org\springframework\boot\spring-boot-starter-json\2.1.5.RELEASE\spring-boot-starter-json-2.1.5.RELEASE.jar;C:\maven\mvn_repo3.0\com\fasterxml\jackson\core\jackson-databind\2.9.8\jackson-databind-2.9.8.jar;C:\maven\mvn_repo3.0\com\fasterxml\jackson\core\jackson-annotations\2.9.0\jackson-annotations-2.9.0.jar;C:\maven\mvn_repo3.0\com\fasterxml\jackson\core\jackson-core\2.9.8\jackson-core-2.9.8.jar;C:\maven\mvn_repo3.0\com\fasterxml\jackson\datatype\jackson-datatype-jdk8\2.9.8\jackson-datatype-jdk8-2.9.8.jar;C:\maven\mvn_repo3.0\com\fasterxml\jackson\datatype\jackson-datatype-jsr310\2.9.8\jackson-datatype-jsr310-2.9.8.jar;C:\maven\mvn_repo3.0\com\fasterxml\jackson\module\jackson-module-parameter-names\2.9.8\jackson-module-parameter-names-2.9.8.jar;C:\maven\mvn_repo3.0\org\springframework\boot\spring-boot-starter-tomcat\2.1.5.RELEASE\spring-boot-starter-tomcat-2.1.5.RELEASE.jar;C:\maven\mvn_repo3.0\org\apache\tomcat\embed\tomcat-embed-core\9.0.19\tomcat-embed-core-9.0.19.jar;C:\maven\mvn_repo3.0\org\apache\tomcat\embed\tomcat-embed-el\9.0.19\tomcat-embed-el-9.0.19.jar;C:\maven\mvn_repo3.0\org\apache\tomcat\embed\tomcat-embed-websocket\9.0.19\tomcat-embed-websocket-9.0.19.jar;C:\maven\mvn_repo3.0\org\hibernate\validator\hibernate-validator\6.0.16.Final\hibernate-validator-6.0.16.Final.jar;C:\maven\mvn_repo3.0\javax\validation\validation-api\2.0.1.Final\validation-api-2.0.1.Final.jar;C:\maven\mvn_repo3.0\org\jboss\logging\jboss-logging\3.3.2.Final\jboss-logging-3.3.2.Final.jar;C:\maven\mvn_repo3.0\com\fasterxml\classmate\1.4.0\classmate-1.4.0.jar;C:\maven\mvn_repo3.0\org\springframework\spring-web\5.1.7.RELEASE\spring-web-5.1.7.RELEASE.jar;C:\maven\mvn_repo3.0\org\springframework\spring-beans\5.1.7.RELEASE\spring-beans-5.1.7.RELEASE.jar;C:\maven\mvn_repo3.0\org\springframework\spring-webmvc\5.1.7.RELEASE\spring-webmvc-5.1.7.RELEASE.jar;C:\maven\mvn_repo3.0\org\springframework\spring-aop\5.1.7.RELEASE\spring-aop-5.1.7.RELEASE.jar;C:\maven\mvn_repo3.0\org\springframework\spring-context\5.1.7.RELEASE\spring-context-5.1.7.RELEASE.jar;C:\maven\mvn_repo3.0\org\springframework\spring-expression\5.1.7.RELEASE\spring-expression-5.1.7.RELEASE.jar;C:\maven\mvn_repo3.0\org\slf4j\slf4j-api\1.7.26\slf4j-api-1.7.26.jar;C:\maven\mvn_repo3.0\org\springframework\spring-core\5.1.7.RELEASE\spring-core-5.1.7.RELEASE.jar;C:\maven\mvn_repo3.0\org\springframework\spring-jcl\5.1.7.RELEASE\spring-jcl-5.1.7.RELEASE.jar;C:\maven\mvn_repo3.0\org\springframework\boot\spring-boot-devtools\2.1.5.RELEASE\spring-boot-devtools-2.1.5.RELEASE.jar;C:\maven\mvn_repo3.0\org\springframework\boot\spring-boot\2.1.5.RELEASE\spring-boot-2.1.5.RELEASE.jar;C:\maven\mvn_repo3.0\org\springframework\boot\spring-boot-autoconfigure\2.1.5.RELEASE\spring-boot-autoconfigure-2.1.5.RELEASE.jar;C:\maven\mvn_repo3.0\com\alibaba\fastjson\1.2.47\fastjson-1.2.47.jar;C:\maven\mvn_repo3.0\com\google\code\gson\gson\2.8.5\gson-2.8.5.jar com.example.demo.thread.AcountTransfer
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-1 says accuire lock
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-1 :: Shamik Mitra says ::200 Debited Success Fully
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-1 :: Samir Mitra says ::200 Credited Success Fully
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-1 :: Shamik Mitra says :: now balance is 19800
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-1 :: Samir Mitra says :: now balance is 10200
    pool-1-thread-2 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-2 says accuire lock
    pool-1-thread-1 says :: Transfer successfull
    pool-1-thread-2 :: Samir Mitra says ::1000 Debited Success Fully
    pool-1-thread-2 :: Shamik Mitra says ::1000 Credited Success Fully
    pool-1-thread-2 :: Samir Mitra says :: now balance is 9200
    pool-1-thread-2 :: Shamik Mitra says :: now balance is 20800
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-2 says :: Transfer successfull
    pool-1-thread-3 says accuire lock
    pool-1-thread-3 :: Shamik Mitra says ::200 Debited Success Fully
    pool-1-thread-3 :: Samir Mitra says ::200 Credited Success Fully
    pool-1-thread-3 :: Shamik Mitra says :: now balance is 20600
    pool-1-thread-3 :: Samir Mitra says :: now balance is 9400
    pool-1-thread-3 says :: Transfer successfull
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-3 says fail to accuire both lock Try again
    pool-1-thread-2 says accuire lock
    pool-1-thread-2 :: Shamik Mitra says ::200 Debited Success Fully
    pool-1-thread-2 :: Samir Mitra says ::200 Credited Success Fully
    pool-1-thread-2 :: Shamik Mitra says :: now balance is 20400
    pool-1-thread-2 :: Samir Mitra says :: now balance is 9600
    pool-1-thread-2 says :: Transfer successfull
    pool-1-thread-3 says accuire lock
    pool-1-thread-3 :: Samir Mitra says ::1000 Debited Success Fully
    pool-1-thread-3 :: Shamik Mitra says ::1000 Credited Success Fully
    pool-1-thread-3 :: Samir Mitra says :: now balance is 8600
    pool-1-thread-3 :: Shamik Mitra says :: now balance is 21400
    pool-1-thread-3 says :: Transfer successfull
    pool-1-thread-1 says accuire lock
    pool-1-thread-1 :: Samir Mitra says ::1000 Debited Success Fully
    pool-1-thread-1 :: Shamik Mitra says ::1000 Credited Success Fully
    pool-1-thread-1 :: Samir Mitra says :: now balance is 7600
    pool-1-thread-1 :: Shamik Mitra says :: now balance is 22400
    pool-1-thread-1 says :: Transfer successfull
    pool-1-thread-2 says accuire lock
    pool-1-thread-2 :: Shamik Mitra says ::200 Debited Success Fully
    pool-1-thread-2 :: Samir Mitra says ::200 Credited Success Fully
    pool-1-thread-2 :: Shamik Mitra says :: now balance is 22200
    pool-1-thread-2 :: Samir Mitra says :: now balance is 7800
    pool-1-thread-2 says :: Transfer successfull
    pool-1-thread-3 says accuire lock
    pool-1-thread-3 :: Samir Mitra says ::1000 Debited Success Fully
    pool-1-thread-3 :: Shamik Mitra says ::1000 Credited Success Fully
    pool-1-thread-3 :: Samir Mitra says :: now balance is 6800
    pool-1-thread-3 :: Shamik Mitra says :: now balance is 23200
    pool-1-thread-3 says :: Transfer successfull

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