【java基础】ThreadLocal的实现原理

【一】：ThreadLocal对象的大体实现原理
===>当前线程对象有一个ThreadLocal.ThreadLocalMap属性。
===>声明的ThreadLocal对象最终存储在当前线程对象的属性ThreadLocalMap集合中。
===>ThreadLocalMap集合中的底层是一个Entry数组。每一个ThreadLocal对象经过属性nextHashCode属性计算出当前对象的hashCode决定存储位置。
===>Entry对象存储ThreadLocal对象和要与当前线程绑定的数据。

【二】：ThreadLocal在springAop的事务管理中的应用案例
===>spring的事务管理器类org.springframework.transaction.support.TransactionSynchronizationManager中就运用ThreadLocal对象绑定当前线程数据库事务信息。
===>org.springframework.core.NamedThreadLocal类
===>TransactionSynchronizationManager中绑定了当前线程的数据源，当前线程是否存在活动中的事务，当前线程在事务执行过程中的观察者事件，当前线程是否是只读事务，当前线程事务的名字，当前线程的事务隔离级别。

public abstract class TransactionSynchronizationManager {

    private static final Log logger = LogFactory.getLog(TransactionSynchronizationManager.class);

    private static final ThreadLocal<Map<Object, Object>> resources =
            new NamedThreadLocal<Map<Object, Object>>("Transactional resources");

    private static final ThreadLocal<Set<TransactionSynchronization>> synchronizations =
            new NamedThreadLocal<Set<TransactionSynchronization>>("Transaction synchronizations");

    private static final ThreadLocal<String> currentTransactionName =
            new NamedThreadLocal<String>("Current transaction name");

    private static final ThreadLocal<Boolean> currentTransactionReadOnly =
            new NamedThreadLocal<Boolean>("Current transaction read-only status");

    private static final ThreadLocal<Integer> currentTransactionIsolationLevel =
            new NamedThreadLocal<Integer>("Current transaction isolation level");

    private static final ThreadLocal<Boolean> actualTransactionActive =
            new NamedThreadLocal<Boolean>("Actual transaction active");

【三】：ThreadLocal在项目中的应用
===>ThreadLocalContext类的代码
===>ThreadLocalContext可以保存当前线程一些需要上下文存储数据。

public class MyThreadContext {

    private Map<Object,Object> myData = new HashMap<Object,Object>();
    
    public void put(Object key, Object message){
        myData.put(key, message);
    }
    
    public Object get(Object key){
        return myData.get(key);
    }
    
    public <T> T get(Object key,Class<T> clazz){
        return (T) myData.get(key);
    }
    

    public static MyThreadContext getContext(){
        return threadContext.get();
    }
    

    public static void clearContext(){
        threadContext.remove();
    }


    private static ThreadLocal<ThreadContext> threadContext = new ThreadLocal<ThreadContext>() {
        protected ThreadContext initialValue() {
            return new ThreadContext();
        }
    };
}

为什么ThreadLocalMap的静态内部类Entry要继承WeakReference<ThreadLocal<?>>？