为什么jdk源码推荐ThreadLocal使用static

ThreadLocal是线程私有变量,本身是解决多线程环境线程安全,可以说单线程实际上没必要使用。既Java
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ThreadLocal是线程私有变量,本身是解决多线程环境线程安全,可以说单线程实际上没必要使用。

既然多线程环境本身不使用static,那么又怎么会线程不安全。所以这个问题本身并不是问题,只是有人没有理解ThreadLocal的真正使用场景,所以有此疑问。

看看jdk源码推荐ThreadLocal使用static吧:

/**
 * This class provides thread-local variables.  These variables differ from
 * their normal counterparts in that each thread that accesses one (via its
 * {@code get} or {@code set} method) has its own, independently initialized
 * copy of the variable.  {@code ThreadLocal} instances are typically private
 * static fields in classes that wish to associate state with a thread (e.g.,
 * a user ID or Transaction ID).
 *
 * <p>For example, the class below generates unique identifiers local to each
 * thread.
 * A thread's id is assigned the first time it invokes {@code ThreadId.get()}
 * and remains unchanged on subsequent calls.
 * <pre>
 * import java.util.concurrent.atomic.AtomicInteger;
 *
 * public class ThreadId {
 *     // Atomic integer containing the next thread ID to be assigned
 *     private static final AtomicInteger nextId = new AtomicInteger(0);
 *
 *     // Thread local variable containing each thread's ID
 *     private static final ThreadLocal&lt;Integer&gt; threadId =
 *         new ThreadLocal&lt;Integer&gt;() {
 *             &#64;Override protected Integer initialValue() {
 *                 return nextId.getAndIncrement();
 *         }
 *     };
 *
 *     // Returns the current thread's unique ID, assigning it if necessary
 *     public static int get() {
 *         return threadId.get();
 *     }
 * }
 * </pre>
 * <p>Each thread holds an implicit reference to its copy of a thread-local
 * variable as long as the thread is alive and the {@code ThreadLocal}
 * instance is accessible; after a thread goes away, all of its copies of
 * thread-local instances are subject to garbage collection (unless other
 * references to these copies exist).
 *
 * @author  Josh Bloch and Doug Lea
 * @since   1.2
 */
public class ThreadLocal<T> {
    /**
     * ThreadLocals rely on per-thread linear-probe hash maps attached
     * to each thread (Thread.threadLocals and
     * inheritableThreadLocals).  The ThreadLocal objects act as keys,
     * searched via threadLocalHashCode.  This is a custom hash code
     * (useful only within ThreadLocalMaps) that eliminates collisions
     * in the common case where consecutively constructed ThreadLocals
     * are used by the same threads, while remaining well-behaved in
     * less common cases.
     */

这个是Josh Bloch和Doug Lea写的,再看看他们的著作:《Java并发编程实战》

再看看jdk中的官方文档: 

已经说的很明白了,再看看其他答案吧:

 

按照ThreadLocal类的定义

此类提供线程局部变量。这些变量与普通变量不同,每个访问一个线程(通过其get或set方法)的线程都有其自己的,独立初始化的变量副本。ThreadLocal实例通常是希望将状态与线程关联的类中的私有静态字段(例如,用户ID或事务ID)。

这意味着说2个线程t1t2执行someBMethod(),它们分别结束设置x1x2(的实例X)。现在,当t1come并执行时,someCMethod()它会获取x1(它是由它自己设置的)和gets 。t2x2

换句话说,只有一个静态实例是安全的ThreadLocal,因为在内部调用时它会执行类似的操作set

 

 

Java 源代码

  1. java.lang.Thread Class包含一个实例变量,如下所示。

    ThreadLocal.ThreadLocalMap threadLocals = null;

因为threadLocals变量是非静态的,所以应用程序中的每个线程(即Thread Class的每个实例)将拥有它自己的threadLocals映射副本

  1. 该映射的当前 ThreadLocal实例,而value是您作为参数传递给ThreadLocal.set()的值。

  2. 当您尝试在内部获取 as值时ThreadLocal.get(),它将从Current Thread的ThreadLocalMap获取。

简而言之,您是从当前线程对象获取&设置值,而不是ThreadLocal对象获取&设置值。

 

本文来源21aspnet,由架构君转载发布,观点不代表Java架构师必看的立场,转载请标明来源出处:https://javajgs.com/archives/2002

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