什么时候应该使用ThreadLocal变量?

它是如何使用的?


当前回答

Threadlocal提供了一种非常简单的零成本实现对象可重用性的方法。

我遇到过这样一种情况,在每次更新通知时,多个线程都在创建可变缓存的映像。

我在每个线程上使用Threadlocal,然后每个线程只需要重置旧映像,然后在每次更新通知时从缓存中再次更新它。

来自对象池的通常可重用对象具有与之相关的线程安全成本,而此方法没有。

其他回答

ThreadLocal will ensure accessing the mutable object by the multiple threads in the non synchronized method is synchronized, means making the mutable object to be immutable within the method. This is achieved by giving new instance of mutable object for each thread try accessing it. So It is local copy to the each thread. This is some hack on making instance variable in a method to be accessed like a local variable. As you aware method local variable is only available to the thread, one difference is; method local variables will not available to the thread once method execution is over where as mutable object shared with threadlocal will be available across multiple methods till we clean it up.

通过定义:

Java中的ThreadLocal类允许您创建这样的变量 只能在同一线程上读写。这样,即使是两个线程 正在执行相同的代码,并且该代码有一个对 变量ThreadLocal,那么两个线程不能看到彼此的线程 ThreadLocal变量。

java中的每个线程都包含ThreadLocalMap。 在哪里

Key = One ThreadLocal object shared across threads.
value = Mutable object which has to be used synchronously, this will be instantiated for each thread.

实现ThreadLocal:

现在为ThreadLocal创建一个包装器类,它将保存如下所示的可变对象(有或没有initialValue())。现在这个包装器的getter和setter将工作于threadlocal实例,而不是可变对象。

如果threadlocal的getter()在线程的threadlocalmap中没有找到任何值;然后它将调用initialValue()来获得它相对于线程的私有副本。

class SimpleDateFormatInstancePerThread {

    private static final ThreadLocal<SimpleDateFormat> dateFormatHolder = new ThreadLocal<SimpleDateFormat>() {

        @Override
        protected SimpleDateFormat initialValue() {
            SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd") {
                UUID id = UUID.randomUUID();
                @Override
                public String toString() {
                    return id.toString();
                };
            };
            System.out.println("Creating SimpleDateFormat instance " + dateFormat +" for Thread : " + Thread.currentThread().getName());
            return dateFormat;
        }
    };

    /*
     * Every time there is a call for DateFormat, ThreadLocal will return calling
     * Thread's copy of SimpleDateFormat
     */
    public static DateFormat getDateFormatter() {
        return dateFormatHolder.get();
    }

    public static void cleanup() {
        dateFormatHolder.remove();
    }
}

现在wrapper.getDateFormatter()将调用threadlocal.get()并检查currentThread。threadLocalMap包含这个(threadlocal)实例。 如果是,返回对应threadlocal实例的值(SimpleDateFormat) 否则使用这个threadlocal实例initialValue()添加映射。

在此可变类上实现线程安全;每个线程都使用自己的可变实例,但使用相同的ThreadLocal实例。意味着所有线程将共享相同的ThreadLocal实例作为key,但不同的SimpleDateFormat实例作为value。

https://github.com/skanagavelu/yt.tech/blob/master/src/ThreadLocalTest.java

缓存,有时你必须计算相同的值很多时间,所以通过存储最后一组输入到一个方法和结果,你可以加快代码。通过使用线程本地存储,您可以避免考虑锁定问题。

从本质上讲,当您需要一个变量的值依赖于当前线程,并且不方便您以其他方式将值附加到线程(例如,子类化线程)。

典型的情况是,其他框架创建了运行代码的线程,例如servlet容器,或者使用ThreadLocal更有意义,因为你的变量“在它的逻辑位置”(而不是挂在thread子类或其他哈希映射中的变量)。

在我的网站上,我有一些关于何时使用ThreadLocal的进一步讨论和示例,您可能也会感兴趣。

有些人主张使用ThreadLocal作为一种方法,在某些并发算法中,当你需要线程号时,将“线程ID”附加到每个线程上(参见Herlihy & Shavit)。在这种情况下,检查你是否真的得到了好处!

Threadlocal提供了一种非常简单的零成本实现对象可重用性的方法。

我遇到过这样一种情况,在每次更新通知时,多个线程都在创建可变缓存的映像。

我在每个线程上使用Threadlocal,然后每个线程只需要重置旧映像,然后在每次更新通知时从缓存中再次更新它。

来自对象池的通常可重用对象具有与之相关的线程安全成本,而此方法没有。

在《Java并发实践》一书中有一个很好的例子。作者(Joshua Bloch)解释了线程限制是实现线程安全的最简单方法之一,而ThreadLocal是维护线程限制的更正式的方法。最后,他还解释了人们如何滥用它作为全局变量。

我已经从提到的书中复制了文本,但代码3.10是缺失的,因为它不太重要,了解ThreadLocal应该在哪里使用。

Thread-local variables are often used to prevent sharing in designs based on mutable Singletons or global variables. For example, a single-threaded application might maintain a global database connection that is initialized at startup to avoid having to pass a Connection to every method. Since JDBC connections may not be thread-safe, a multithreaded application that uses a global connection without additional coordination is not thread-safe either. By using a ThreadLocal to store the JDBC connection, as in ConnectionHolder in Listing 3.10, each thread will have its own connection. ThreadLocal is widely used in implementing application frameworks. For example, J2EE containers associate a transaction context with an executing thread for the duration of an EJB call. This is easily implemented using a static Thread-Local holding the transaction context: when framework code needs to determine what transaction is currently running, it fetches the transaction context from this ThreadLocal. This is convenient in that it reduces the need to pass execution context information into every method, but couples any code that uses this mechanism to the framework. It is easy to abuse ThreadLocal by treating its thread confinement property as a license to use global variables or as a means of creating “hidden” method arguments. Like global variables, thread-local variables can detract from reusability and introduce hidden couplings among classes, and should therefore be used with care.