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版本一:
// Single threaded version
class Foo { private static Helper helper = null; private Helper(){ } public static Helper getHelper() { if (helper == null) helper = new Helper(); return helper; } // other functions and members... }
版本二:
// Correct but possibly expensive multithreaded version
class Foo { private Helper helper = null; public synchronized Helper getHelper() { if (helper == null) helper = new Helper(); return helper; } // other functions and members... }
版本三(DCL):
// Broken multithreaded version
// "Double-Checked Locking" idiom class Foo { private Helper helper = null; public Helper getHelper() { if (helper == null) { synchronized(this) { if (helper == null) { helper = new Helper(); } } } return helper; } // other functions and members... }
DCL还是会失效,最重要的2个原因如下:
1, 编译器优化了程序指令, 以加快cpu处理速度.
2, 多核cpu动态调整指令顺序, 以加快并行运算能力.
解决一:
// Works with acquire/release semantics for volatile
// Broken under Java 1.4 and earlier semantics for volatile
class Foo {
private volatile Helper helper = null;
public Helper getHelper() {
if (helper == null) {
synchronized(this) {
if (helper == null)
helper = new Helper();
}
}
return helper;
}
// other functions and members...
}
解决二:
public class Foo {
// 似有静态内部类, 只有当有引用时, 该类才会被装载
private static class LazyFoo {
public static Foo foo = new Foo();
}
 
public static Foo getInstance() {
return LazyFoo.foo;
}
}
 
参考:
http://kenwublog.com/explain-java-memory-model-in-detail(涉及了很多Java底层的东西,值得一看)
http://en.wikipedia.org/wiki/Double-checked_locking
posted on 2010-01-03 19:55 calvin 阅读(211) 评论(0)  编辑  收藏 所属分类: Java

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