处理多个线程使用的实例中的状态(Handling state in an instance that is used by multiple threads)

 让我们考虑遵循可变的Java类（取自Brian Goetz的“Java Concurrency in Practice”）： 
 
@ThreadSafe
public class SynchronizedInteger {
  @GuardedBy("this") private int value;

  public synchronized int get() { return value; }
  public synchronized void set(int value) { this.value = value; }
}
 
 假设此类具有多个线程使用的单个实例，添加到类中的以下哪些方法不会破坏其线程安全性？ 
 
 解决方案＃1： 
 
public boolean isPositive() { return value > 0; }
 
 解决方案＃2： 
 
public synchronized boolean isPositive() { return value > 0; }
 
 解决方案＃3： 
 
public boolean isPositive() { return get() > 0; }
 
 我的理解是，为了确保可见性，我需要确保在CPU /核心缓存中正确刷新字段值 ，因此解决方案＃2和＃3是可行的方法。 谁能证实这一点？ 
 
 第二个问题：让我们假设我们有一个Java Spring单例，它在私有字段中保存了一些状态。 这个单例由一些请求范围的bean使用。 现在，每个请求都有自己的线程（使用它自己的调用堆栈，但共享堆）。 由于单例的状态驻留在堆上，因此单例中定义的任何方法都应该以某种方式同步访问它的私有成员（=其状态）以确保可见性，例如使用lock，volatile，ReentrantLock等。我得到对的？ 

Let's consider following mutable Java class (taken from "Java Concurrency in Practice" by Brian Goetz): 
 
@ThreadSafe
public class SynchronizedInteger {
  @GuardedBy("this") private int value;

  public synchronized int get() { return value; }
  public synchronized void set(int value) { this.value = value; }
}
 
Assuming that this class has a single instance that is used by multiple threads, which of the following methods added to the class does NOT break its thread-safety?
 
Solution #1:
 
public boolean isPositive() { return value > 0; }
 
Solution #2:
 
public synchronized boolean isPositive() { return value > 0; }
 
Solution #3:
 
public boolean isPositive() { return get() > 0; }
 
My understanding is that, in order to ensure visibility, I need to make sure that the field value is properly refreshed in the CPU/core cache, so Solution #2 and #3 are the way to go. Can anyone confirm this?
 
Second question: Let's assume we have a Java Spring singleton that has some state held in a private field. This singleton is used by some request-scoped beans. Now, each request has it's own thread (with it's own call stack, but shared heap). Since the state of the singleton resides on the heap, any method defined in the singleton should somehow synchronize access to it's private member (= its state) in order to ensure the visibility, e.g. using lock, volatile, ReentrantLock, etc. Do I get it right?

原文：https://stackoverflow.com/questions/37845710