Happen before 法则是在JDK哪个版本加入，为什么jdk1.5后支持双重检查锁？
走同样的路，发现不同的人生
JDK1.5 adopts the new memory model JSR133, which defines the happen before principle.
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JDK1.5 adopts the new memory model JSR133, which defines the happen before principle.