threadlocal原理及常用应用场景2

来源：互联网发布：化工公司工艺软件编辑：程序博客网时间：2024/06/06 03:37

最近项目中遇到如下的场景：在执行数据迁移时，需要按照用户粒度加锁，因此考虑使用排他锁，迁移工具和业务服务属于两个服务，因此需要使用分布式锁。

我们使用缓存（Tair或者Redis）实现分布式锁，具体代码如下：

@Service
public class Locker {
@Resource(name = "tairClientUtil")
private TairClientUtil tairClientUtil;
private ThreadLocal<Long> lockerBeanThreadLocal = new ThreadLocal<>();
public void init(long userid) {
lockerBeanThreadLocal.remove();
lockerBeanThreadLocal.set(userid);
}
public void updateLock() {
String lockKey = Constants.MIGRATION_PREFIX + lockerBeanThreadLocal.get();
tairClientUtil.incr(lockKey, Constants.COUNT_EXPIRE);
}
public void invalidLock() {
String lockKey = Constants.MIGRATION_PREFIX + lockerBeanThreadLocal.get();
tairClientUtil.invalid(lockKey);
}
}
作者：杜琪
链接：http://www.jianshu.com/p/cadd53f063b9

因为每个线程可能携带不同的userid发起请求，因此在这里使用ThreadLocal变量存放userid，使得每个线程都有一份自己的副本。

参考官方文档：ThreadLocal的用法，这个类提供“thread-local”变量，这些变量与线程的局部变量不同，每个线程都保存一份改变量的副本，可以通过get或者set方法访问。如果开发者希望将类的某个静态变量（user ID或者transaction ID）与线程状态关联，则可以考虑使用ThreadLocal。

举个例子，下面的类为每个线程生成不同的ID，当某个线程第一次调用Thread.get()时，会为该线程赋予一个ID，并且在后续的调用中不再改变。

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<Integer> threadId =
new ThreadLocal<Integer>() {
@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();
}
}
每个线程会“隐式”包含一份thread-local变量的副本，只要线程还处于活跃状态，就可以访问该变量；当线程停止后，如果没有其他线程持有该thread-local变量，则该变量的副本会提交给垃圾回收器。
/**
* {@link ThreadLocal} subclass that exposes a specified name
* as {@link #toString()} result (allowing for introspection).
*
* @author Juergen Hoeller
* @since 2.5.2
* @see NamedInheritableThreadLocal
*/
public class NamedThreadLocal<T> extends ThreadLocal<T> {
private final String name;
/**
* Create a new NamedThreadLocal with the given name.
* @param name a descriptive name for this ThreadLocal
*/
public NamedThreadLocal(String name) {
Assert.hasText(name, "Name must not be empty");
this.name = name;
}
@Override
public String toString() {
return this.name;
}
}
XmlBeanDefinitionReader.java
private final ThreadLocal<Set<EncodedResource>> resourcesCurrentlyBeingLoaded =
new NamedThreadLocal<Set<EncodedResource>>("XML bean definition resources currently being loaded");
/**
* Load bean definitions from the specified XML file.
* @param encodedResource the resource descriptor for the XML file,
* allowing to specify an encoding to use for parsing the file
* @return the number of bean definitions found
* @throws BeanDefinitionStoreException in case of loading or parsing errors
*/
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
Assert.notNull(encodedResource, "EncodedResource must not be null");
if (logger.isInfoEnabled()) {
logger.info("Loading XML bean definitions from " + encodedResource.getResource());
}
Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
if (currentResources == null) {
currentResources = new HashSet<EncodedResource>(4);
this.resourcesCurrentlyBeingLoaded.set(currentResources);
}
if (!currentResources.add(encodedResource)) {
throw new BeanDefinitionStoreException(
"Detected cyclic loading of " + encodedResource + " - check your import definitions!");
}
try {
InputStream inputStream = encodedResource.getResource().getInputStream();
try {
InputSource inputSource = new InputSource(inputStream);
if (encodedResource.getEncoding() != null) {
inputSource.setEncoding(encodedResource.getEncoding());
}
return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
}
finally {
inputStream.close();
}
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(
"IOException parsing XML document from " + encodedResource.getResource(), ex);
}
finally {
currentResources.remove(encodedResource);
if (currentResources.isEmpty()) {
this.resourcesCurrentlyBeingLoaded.remove();
}
}
}

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