开发过程中发现,同一个ThreadLocal变量在父线程中被设置值后,在子线程中是获取不到的,经过查询学习,发现有InheritableThreadLocal这么一个神奇的类,提供了一个特性: 让子线程可以访问父线程中设置的本地变量,我们来看下源码:
public class InheritableThreadLocal<T> extends ThreadLocal<T> {
/**
* Computes the child's initial value for this inheritable thread-local
* variable as a function of the parent's value at the time the child
* thread is created. This method is called from within the parent
* thread before the child is started.
* <p>
* This method merely returns its input argument, and should be overridden
* if a different behavior is desired.
*
* @param parentValue the parent thread's value
* @return the child thread's initial value
*/
protected T childValue(T parentValue) {
return parentValue;
}
/**
* Get the map associated with a ThreadLocal.
*
* @param t the current thread
*/
ThreadLocalMap getMap(Thread t) {
return t.inheritableThreadLocals;
}
/**
* Create the map associated with a ThreadLocal.
*
* @param t the current thread
* @param firstValue value for the initial entry of the table.
*/
void createMap(Thread t, T firstValue) {
t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
}
如上代码所示,InheritableThreadLocal继承了ThreadLocal,并重写了3个方法.
createMap被重写,当第一次调用set方法的时候,创建的是当前线程的inheritableThreadLocals 变量的实例而不再是ThreadLocals.
get方法也是同样.
我们看下childValue(T parentValue)何时被执行,以及如何让子线程可以访问父线程的本地变量. 打开Thread类的默认构造函数,代码如下:
/**
* Initializes a Thread with the current AccessControlContext.
* @see #init(ThreadGroup,Runnable,String,long,AccessControlContext,boolean)
*/
private void init(ThreadGroup g, Runnable target, String name,
long stackSize) {
init(g, target, name, stackSize, null, true);
}
/**
* Initializes a Thread.
*
* @param g the Thread group
* @param target the object whose run() method gets called
* @param name the name of the new Thread
* @param stackSize the desired stack size for the new thread, or
* zero to indicate that this parameter is to be ignored.
* @param acc the AccessControlContext to inherit, or
* AccessController.getContext() if null
* @param inheritThreadLocals if {@code true}, inherit initial values for
* inheritable thread-locals from the constructing thread
*/
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc,
boolean inheritThreadLocals) {
if (name == null) {
throw new NullPointerException("name cannot be null");
}
this.name = name;
Thread parent = currentThread();
SecurityManager security = System.getSecurityManager();
if (g == null) {
/* Determine if it's an applet or not */
/* If there is a security manager, ask the security manager
what to do. */
if (security != null) {
g = security.getThreadGroup();
}
/* If the security doesn't have a strong opinion of the matter
use the parent thread group. */
if (g == null) {
g = parent.getThreadGroup();
}
}
/* checkAccess regardless of whether or not threadgroup is
explicitly passed in. */
g.checkAccess();
/*
* Do we have the required permissions?
*/
if (security != null) {
if (isCCLOverridden(getClass())) {
security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
}
}
g.addUnstarted();
this.group = g;
this.daemon = parent.isDaemon();
this.priority = parent.getPriority();
if (security == null || isCCLOverridden(parent.getClass()))
this.contextClassLoader = parent.getContextClassLoader();
else
this.contextClassLoader = parent.contextClassLoader;
this.inheritedAccessControlContext =
acc != null ? acc : AccessController.getContext();
this.target = target;
setPriority(priority);
// (1)如果父线程的inheritableThreadLocals 不为null
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
// (2)设置子线程中的inheritableThreadLocals 变量
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
/* Stash the specified stack size in case the VM cares */
this.stackSize = stackSize;
/* Set thread ID */
tid = nextThreadID();
}
如上代码在创建线程时,会调用init方法
Thread parent = currentThread();获取了当前线程(这里指main函数所在线程,也就是父线程),然后(1)判断了父线程的属性输不是null,如果不是null则会执行代码(2)
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
return new ThreadLocalMap(parentMap);
}
可以看到, 在createInheritedMap内部使用父线程的inheritableThreadLocals变量作为构造函数创建了一个新的ThreadLocalMap变量,然后赋值给了子线程的inheritableThreadLocals变量,下面我们看下在ThreadLocalMap构造函数内部做了些什么:
private ThreadLocalMap(ThreadLocalMap parentMap) {
Entry[] parentTable = parentMap.table;
int len = parentTable.length;
setThreshold(len);
table = new Entry[len];
for (int j = 0; j < len; j++) {
Entry e = parentTable[j];
if (e != null) {
@SuppressWarnings("unchecked")
ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
if (key != null) {
Object value = key.childValue(e.value);
Entry c = new Entry(key, value);
int h = key.threadLocalHashCode & (len - 1);
while (table[h] != null)
h = nextIndex(h, len);
table[h] = c;
size++;
}
}
}
}
在该构造函数内部把父线程的inheritableThreadLocals成员变量的值复制到新的ThreadLocalMap对象中.其中Object value = key.childValue(e.value);调用了inheritableThreadLocal类重写的方法.
这种办法对中间件需要把统一的id追踪的整个调用链路记录下来很有好处, 我觉得大众点评的cat可能就是这么做的,虽然我还没有研究.
与君共勉...
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