什么时候应该使用ThreadLocal变量?
它是如何使用的?
什么时候应该使用ThreadLocal变量?
它是如何使用的?
当前回答
ThreadLocal是JVM专门提供的功能,仅为线程提供隔离的存储空间。与实例作用域变量的值一样,变量只能绑定到类的给定实例。每个对象都有其唯一的值,它们不能看到彼此的值。ThreadLocal变量的概念也是如此,在对象实例的意义上,它们是线程的本地线程,除了创建它的线程之外,其他线程看不到它。在这里看到的
import java.util.concurrent.atomic.AtomicInteger;
import java.util.stream.IntStream;
public class ThreadId {
private static final AtomicInteger nextId = new AtomicInteger(1000);
// Thread local variable containing each thread's ID
private static final ThreadLocal<Integer> threadId = ThreadLocal.withInitial(() -> nextId.getAndIncrement());
// Returns the current thread's unique ID, assigning it if necessary
public static int get() {
return threadId.get();
}
public static void main(String[] args) {
new Thread(() -> IntStream.range(1, 3).forEach(i -> {
System.out.println(Thread.currentThread().getName() + " >> " + new ThreadId().get());
})).start();
new Thread(() -> IntStream.range(1, 3).forEach(i -> {
System.out.println(Thread.currentThread().getName() + " >> " + new ThreadId().get());
})).start();
new Thread(() -> IntStream.range(1, 3).forEach(i -> {
System.out.println(Thread.currentThread().getName() + " >> " + new ThreadId().get());
})).start();
}
}
其他回答
可以使用threadlocal变量的两个用例- 1-当我们需要将状态与线程关联时(例如,用户ID或事务ID)。这通常发生在web应用程序中,每个发送到servlet的请求都有一个与之关联的唯一transactionID。
// This class will provide a thread local variable which
// will provide a unique ID for each thread
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<Integer> threadId =
ThreadLocal.<Integer>withInitial(()-> {return nextId.getAndIncrement();});
// Returns the current thread's unique ID, assigning it if necessary
public static int get() {
return threadId.get();
}
}
注意,这里的withInitial方法是使用lambda表达式实现的。 2-另一个用例是当我们想要一个线程安全的实例时,我们不想使用同步,因为同步的性能成本更高。其中一种情况是使用SimpleDateFormat。由于SimpleDateFormat不是线程安全的,所以我们必须提供机制使其线程安全。
public class ThreadLocalDemo1 implements Runnable {
// threadlocal variable is created
private static final ThreadLocal<SimpleDateFormat> dateFormat = new ThreadLocal<SimpleDateFormat>(){
@Override
protected SimpleDateFormat initialValue(){
System.out.println("Initializing SimpleDateFormat for - " + Thread.currentThread().getName() );
return new SimpleDateFormat("dd/MM/yyyy");
}
};
public static void main(String[] args) {
ThreadLocalDemo1 td = new ThreadLocalDemo1();
// Two threads are created
Thread t1 = new Thread(td, "Thread-1");
Thread t2 = new Thread(td, "Thread-2");
t1.start();
t2.start();
}
@Override
public void run() {
System.out.println("Thread run execution started for " + Thread.currentThread().getName());
System.out.println("Date formatter pattern is " + dateFormat.get().toPattern());
System.out.println("Formatted date is " + dateFormat.get().format(new Date()));
}
}
文档说得很好:“每个访问[线程局部变量]的线程(通过它的get或set方法)都有它自己的、独立初始化的变量副本”。
当每个线程必须有自己的某个副本时,可以使用一个。默认情况下,数据在线程之间共享。
自Java 8发布以来,有更多的声明性方法来初始化ThreadLocal:
ThreadLocal<String> local = ThreadLocal.withInitial(() -> "init value");
在Java 8发布之前,你必须做以下事情:
ThreadLocal<String> local = new ThreadLocal<String>(){
@Override
protected String initialValue() {
return "init value";
}
};
此外,如果用于ThreadLocal的类的实例化方法(构造函数,工厂方法)不接受任何参数,您可以简单地使用方法引用(在Java 8中引入):
class NotThreadSafe {
// no parameters
public NotThreadSafe(){}
}
ThreadLocal<NotThreadSafe> container = ThreadLocal.withInitial(NotThreadSafe::new);
注意: 计算是惰性的,因为你传递的java.util.function.Supplier lambda只在调用ThreadLocal#get但value之前没有计算时才会计算。
在Java中,如果您有一个每个线程都可以变化的数据,那么您可以选择将该数据传递给每个需要(或可能需要)它的方法,或者将该数据与线程关联。如果你的所有方法都需要传递一个公共的“上下文”变量,那么到处传递数据可能是可行的。
如果不是这样,您可能不希望用额外的参数来打乱方法签名。在非线程环境中,可以使用Java中等价的全局变量来解决这个问题。在线程词中,与全局变量等价的是线程局部变量。
ThreadLocal in Java had been introduced on JDK 1.2 but was later generified in JDK 1.5 to introduce type safety on ThreadLocal variable. ThreadLocal can be associated with Thread scope, all the code which is executed by Thread has access to ThreadLocal variables but two thread can not see each others ThreadLocal variable. Each thread holds an exclusive copy of ThreadLocal variable which becomes eligible to Garbage collection after thread finished or died, normally or due to any Exception, Given those ThreadLocal variable doesn't have any other live references. ThreadLocal variables in Java are generally private static fields in Classes and maintain its state inside Thread.
阅读更多:Java示例程序和教程中的ThreadLocal