java 对象锁

2023年5月25日 215次阅读来源: yyyyy5101

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(); } }

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(); } }

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
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