为什么 Java EE 单例会话 bean 的字段不必是易变的？

Question

这是 Java EE 教程中的单例会话 bean 示例：

@ConcurrencyManagement(ConcurrencyManagementType.CONTAINER) 
@Singleton
public class ExampleSingletonBean {

    private String state;

    @Lock(LockType.READ)
    public String getState() {
        return state;
    }

    @Lock(LockType.WRITE)
    public void setState(String newState) {
        state = newState;
    }
}

private String state 字段怎么可能不必是可变的？应用程序服务器如何保证此引用永远不会为不同的、同时发生的 HTTP 请求在线程本地缓存？

Answer 1

如 here 所述，当您使用 CONTAINER 时，将使用 Lock。

现在锁本身以某种方式模拟了 volatile，可以阅读 here:

All Lock implementations must enforce the same memory synchronization semantics as provided by the built-in monitor lock, as described in The Java Language Specification, Third Edition (17.4 Memory Model):

A successful lock operation has the same memory synchronization effects as a successful Lock action.

A successful unlock operation has the same memory synchronization effects as a successful Unlock action.

换句话说，隐式锁是"volatile"（也许不一定是关键字，而是实现）。

为什么 Java EE 单例会话 bean 的字段不必是易变的？

Why Java EE singleton session bean's fields don't have to be volatile?

jakarta-ee

singleton

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