Calvin's Tech Space

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還是會失效，最重要的2個原因如下:

1, 編譯器優化了程序指令, 以加快cpu處理速度.

2, 多核cpu動態調整指令順序, 以加快并行運算能力.

解決一：

// 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...
}

解決二：
public class Foo {
// 似有靜態內部類, 只有當有引用時, 該類才會被裝載
private static class LazyFoo {
public static Foo foo = new Foo();
}
 
public static Foo getInstance() {
return LazyFoo.foo;
}
}

 
參考：
http://kenwublog.com/explain-java-memory-model-in-detail（涉及了很多Java底層的東西，值得一看）