java 对象锁

在并发环境下,解决共享资源冲突问题时,可以考虑使用锁机制。
1.
对象的锁

所有对象都自动含有单一的锁。
JVM
负责跟踪对象被加锁的次数。如果一个对象被解锁,其计数变为
0
。在任务(线程)第一次给对象加锁的时候,计数变为
1
。每当这个相同的任务(线程)在此对象上获得锁时,计数会递增。
只有首先获得锁的任务(线程)才能继续获取该对象上的多个锁。
每当任务离开一个
synchronized
方法,计数递减,当计数为
0
的时候,锁被完全释放,此时别的任务就可以使用此资源。
2.synchronized
同步块

2.1
同步到单一对象锁

当使用同步块时,如果方法下的同步块都同步到一个对象上的锁,则所有的任务(线程)只能互斥的进入这些同步块。
Resource1.java
演示了三个线程(包括
main
线程)试图进入某个类的三个不同的方法的同步块中,虽然这些同步块处在不同的方法中,但由于是同步到同一个对象(当前对象
 synchronized
 (
this
)
),所以对它们的方法依然是互斥的。
Resource1.java

package  com.zj.lock; import  java.util.concurrent.TimeUnit;   public   class  Resource1 {      public   void  f() {         // other operations should not be locked…        System. out .println(Thread.currentThread().getName()                “:not synchronized in f()” );         synchronized  ( this ) {             for  ( int  i = 0; i < 5; i++) {               System. out .println(Thread.currentThread().getName()                       “:synchronized in f()” );                try  {                   TimeUnit. SECONDS .sleep(3);                catch  (InterruptedException e) {                   e.printStackTrace();               }            }        }     }        public   void  g() {         // other operations should not be locked…        System. out .println(Thread.currentThread().getName()                “:not synchronized in g()” );         synchronized  ( this ) {             for  ( int  i = 0; i < 5; i++) {               System. out .println(Thread.currentThread().getName()                       “:synchronized in g()” );                try  {                   TimeUnit. SECONDS .sleep(3);                catch  (InterruptedException e) {                   e.printStackTrace();               }            }        }     }        public   void  h() {         // other operations should not be locked…        System. out .println(Thread.currentThread().getName()                “:not synchronized in h()” );         synchronized  ( this ) {             for  ( int  i = 0; i < 5; i++) {               System. out .println(Thread.currentThread().getName()                       “:synchronized in h()” );                try  {                   TimeUnit. SECONDS .sleep(3);                catch  (InterruptedException e) {                   e.printStackTrace();               }            }        }     }        public   static   void  main(String[] args) {         final  Resource1 rs =  new  Resource1();           new  Thread() {             public   void  run() {               rs.f();            }        }.start();           new  Thread() {             public   void  run() {               rs.g();            }        }.start();          rs.h();     } }

结果:
Thread-0:not synchronized in f()

Thread-0:synchronized in f()

main:not synchronized in h()

Thread-1:not synchronized in g()

Thread-0:synchronized in f()

Thread-0:synchronized in f()

Thread-0:synchronized in f()

Thread-0:synchronized in f()

Thread-1:synchronized in g()

Thread-1:synchronized in g()

Thread-1:synchronized in g()

Thread-1:synchronized in g()

Thread-1:synchronized in g()

main:synchronized in h()

main:synchronized in h()

main:synchronized in h()

main:synchronized in h()

main:synchronized in h()
2.2 
同步到多个对象锁

Resource1.java
演示了三个线程(包括
main
线程)试图进入某个类的三个不同的方法的同步块中,这些同步块处在不同的方法中,并且是同步到三个不同的对象(
synchronized
 (
this
)

synchronized
(
syncObject1
)

synchronized
 (
syncObject2
)
),所以对它们的方法中的临界资源访问是独立的。
Resource2.java

package  com.zj.lock; import  java.util.concurrent.TimeUnit;   public   class  Resource2 {      private  Object  syncObject1  =  new  Object();      private  Object  syncObject2  =  new  Object();        public   void  f() {         // other operations should not be locked…        System. out .println(Thread.currentThread().getName()                “:not synchronized in f()” );         synchronized  ( this ) {             for  ( int  i = 0; i < 5; i++) {               System. out .println(Thread.currentThread().getName()                       “:synchronized in f()” );                try  {                   TimeUnit. SECONDS .sleep(3);                catch  (InterruptedException e) {                   e.printStackTrace();               }            }        }     }        public   void  g() {         // other operations should not be locked…        System. out .println(Thread.currentThread().getName()                “:not synchronized in g()” );         synchronized  ( syncObject1 ) {             for  ( int  i = 0; i < 5; i++) {               System. out .println(Thread.currentThread().getName()                       “:synchronized in g()” );                try  {                   TimeUnit. SECONDS .sleep(3);                catch  (InterruptedException e) {                   e.printStackTrace();               }            }        }     }        public   void  h() {         // other operations should not be locked…        System. out .println(Thread.currentThread().getName()                “:not synchronized in h()” );         synchronized  ( syncObject2 ) {             for  ( int  i = 0; i < 5; i++) {               System. out .println(Thread.currentThread().getName()                       “:synchronized in h()” );                try  {                   TimeUnit. SECONDS .sleep(3);                catch  (InterruptedException e) {                   e.printStackTrace();               }            }        }     }        public   static   void  main(String[] args) {         final  Resource2 rs =  new  Resource2();           new  Thread() {             public   void  run() {               rs.f();            }        }.start();           new  Thread() {             public   void  run() {               rs.g();            }        }.start();          rs.h();     } }

结果:
Thread-0:not synchronized in f()

Thread-0:synchronized in f()

main:not synchronized in h()

main:synchronized in h()

Thread-1:not synchronized in g()

Thread-1:synchronized in g()

Thread-0:synchronized in f()

main:synchronized in h()

Thread-1:synchronized in g()

Thread-0:synchronized in f()

main:synchronized in h()

Thread-1:synchronized in g()

Thread-0:synchronized in f()

main:synchronized in h()

Thread-1:synchronized in g()

Thread-0:synchronized in f()

main:synchronized in h()

Thread-1:synchronized in g()
3.Lock
对象锁

除了使用
synchronized
外,还可以使用
Lock
对象来创建临界区。
Resource3.java
的演示效果同
Resource1.java

Resource4.java
的演示效果同
Resource2.java


Resource3.java

package  com.zj.lock; import  java.util.concurrent.TimeUnit; import  java.util.concurrent.locks.Lock; import  java.util.concurrent.locks.ReentrantLock;   public   class  Resource3 {      private  Lock  lock  =  new  ReentrantLock();        public   void  f() {         // other operations should not be locked…        System. out .println(Thread.currentThread().getName()                “:not synchronized in f()” );         lock .lock();         try  {             for  ( int  i = 0; i < 5; i++) {               System. out .println(Thread.currentThread().getName()                       “:synchronized in f()” );                try  {                   TimeUnit. SECONDS .sleep(3);                catch  (InterruptedException e) {                   e.printStackTrace();               }            }         finally  {             lock .unlock();        }     }        public   void  g() {         // other operations should not be locked…        System. out .println(Thread.currentThread().getName()                “:not synchronized in g()” );         lock .lock();         try  {             for  ( int  i = 0; i < 5; i++) {               System. out .println(Thread.currentThread().getName()                       “:synchronized in g()” );                try  {                   TimeUnit. SECONDS .sleep(3);                catch  (InterruptedException e) {                   e.printStackTrace();               }            }         finally  {             lock .unlock();        }     }        public   void  h() {         // other operations should not be locked…        System. out .println(Thread.currentThread().getName()                “:not synchronized in h()” );         lock .lock();         try  {             for  ( int  i = 0; i < 5; i++) {               System. out .println(Thread.currentThread().getName()                       “:synchronized in h()” );                try  {                   TimeUnit. SECONDS .sleep(3);                catch  (InterruptedException e) {                   e.printStackTrace();               }            }         finally  {             lock .unlock();        }     }        public   static   void  main(String[] args) {         final  Resource3 rs =  new  Resource3();           new  Thread() {             public   void  run() {               rs.f();            }        }.start();           new  Thread() {             public   void  run() {               rs.g();            }        }.start();          rs.h();     } }

结果:

Thread-0:not synchronized in f()

Thread-0:synchronized in f()

main:not synchronized in h()

Thread-1:not synchronized in g()

Thread-0:synchronized in f()

Thread-0:synchronized in f()

Thread-0:synchronized in f()

Thread-0:synchronized in f()

main:synchronized in h()

main:synchronized in h()

main:synchronized in h()

main:synchronized in h()

main:synchronized in h()

Thread-1:synchronized in g()

Thread-1:synchronized in g()

Thread-1:synchronized in g()

Thread-1:synchronized in g()

Thread-1:synchronized in g()
Resource4.java

package  com.zj.lock; import  java.util.concurrent.TimeUnit; import  java.util.concurrent.locks.Lock; import  java.util.concurrent.locks.ReentrantLock;   public   class  Resource4 {      private  Lock  lock1  =  new  ReentrantLock();      private  Lock  lock2  =  new  ReentrantLock();      private  Lock  lock3  =  new  ReentrantLock();        public   void  f() {         // other operations should not be locked…        System. out .println(Thread.currentThread().getName()                “:not synchronized in f()” );         lock1 .lock();         try  {             for  ( int  i = 0; i < 5; i++) {               System. out .println(Thread.currentThread().getName()                       “:synchronized in f()” );                try  {                   TimeUnit. SECONDS .sleep(3);                catch  (InterruptedException e) {                   e.printStackTrace();               }            }         finally  {             lock1 .unlock();        }     }        public   void  g() {         // other operations should not be locked…        System. out .println(Thread.currentThread().getName()                “:not synchronized in g()” );         lock2 .lock();         try  {             for  ( int  i = 0; i < 5; i++) {               System. out .println(Thread.currentThread().getName()                       “:synchronized in g()” );                try  {                   TimeUnit. SECONDS .sleep(3);                catch  (InterruptedException e) {                   e.printStackTrace();               }            }         finally  {             lock2 .unlock();        }     }        public   void  h() {         // other operations should not be locked…        System. out .println(Thread.currentThread().getName()                “:not synchronized in h()” );         lock3 .lock();         try  {             for  ( int  i = 0; i < 5; i++) {               System. out .println(Thread.currentThread().getName()                       “:synchronized in h()” );                try  {                   TimeUnit. SECONDS .sleep(3);                catch  (InterruptedException e) {                   e.printStackTrace();               }            }         finally  {             lock3 .unlock();        }     }        public   static   void  main(String[] args) {         final  Resource4 rs =  new  Resource4();           new  Thread() {             public   void  run() {               rs.f();            }        }.start();           new  Thread() {             public   void  run() {               rs.g();            }        }.start();          rs.h();     } }

结果:
Thread-0:not synchronized in f()

Thread-0:synchronized in f()

main:not synchronized in h()

main:synchronized in h()

Thread-1:not synchronized in g()

Thread-1:synchronized in g()

Thread-0:synchronized in f()

main:synchronized in h()

Thread-1:synchronized in g()

Thread-0:synchronized in f()

main:synchronized in h()

Thread-1:synchronized in g()

Thread-0:synchronized in f()

main:synchronized in h()

Thread-1:synchronized in g()

Thread-0:synchronized in f()

main:synchronized in h()

Thread-1:synchronized in g()






synchronized和lock的区别:


Lock 的锁定是通过代码实现的,而 synchronized 是在 JVM 层面上实现的
synchronized 在锁定时如果方法块抛出异常,JVM 会自动将锁释放掉,不会因为出了异常没有释放锁造成线程死锁。但是 Lock 的话就享受不到 JVM 带来自动的功能,出现异常时必须在 finally 将锁释放掉,否则将会引起死锁。


在资源竞争不是很激烈的情况下,偶尔会有同步的情形下,synchronized是很合适的。原因在于,编译程序通常会尽可能的进行优化synchronize,另外可读性非常好,不管用没用过5.0多线程包的程序员都能理解。 

ReentrantLock: 
ReentrantLock提供了多样化的同步,比如有时间限制的同步,可以被Interrupt的同步(synchronized的同步是不能Interrupt的)等。在资源竞争不激烈的情形下,性能稍微比synchronized差点点。但是当同步非常激烈的时候,synchronized的性能一下子能下降好几十倍。而ReentrantLock确还能维持常态。 

Atomic: 
和上面的类似,不激烈情况下,性能比synchronized略逊,而激烈的时候,也能维持常态。激烈的时候,Atomic的性能会优于ReentrantLock一倍左右。但是其有一个缺点,就是只能同步一个值,一段代码中只能出现一个Atomic的变量,多于一个同步无效。因为他不能在多个Atomic之间同步。 





    原文作者:yyyyy5101
    原文地址: http://www.cnblogs.com/yyyyy5101/archive/2011/07/20/2112157.html
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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