介绍 InheritableThreadLocal 之前,假设对 ThreadLocal 已经有了一定的理解,比如基本概念,原理,如果没有,可以自行百度java多线程 ThreadLocal了解原理。
这里再复习下 ThreadLocal 的原理,因为会对 InheritableThreadLocal 的理解 有重大的帮助:
threadLocal.set(new Integer(123));
,我们就会在这个线程中的 threadLocals 属性中放入一个键值对,key 是 这个 threadLocal.set(new Integer(123)); 的 threadlocal,value 就是值。threadlocal.get()
方法的时候,首先会根据这个线程得到这个线程的 threadLocals 属性,然后由于这个属性放的是键值对,我们就可以根据键 threadlocal 拿到值。 注意,这时候这个键 threadlocal 和 我们 set 方法的时候的那个键 threadlocal 是一样的,所以我们能够拿到相同的值。Ps:如果这个原理没搞清楚,那么下文估计有比较难理解,所以建议完完全全搞懂这个原理
从上面的介绍我们可以知道,我们其实是根据 Thread.currentThread(),拿到该线程的 threadlocals,从而进一步得到我们之前预先 set 好的值。那么如果我们新开一个线程,这个时候,由于 Thread.currentThread() 已经变了,从而导致获得的 threadlocals 不一样,我们之前并没有在这个新的线程的 threadlocals 中放入值,那么我就再通过 threadlocal.get()方法 是不可能拿到值的。例如如下代码:
public class Test {
public static ThreadLocal threadLocal = new ThreadLocal();
public static void main(String args[]){
threadLocal.set(new Integer(123));
Thread thread = new MyThread();
thread.start();
System.out.println("main = " + threadLocal.get());
}
static class MyThread extends Thread{
@Override
public void run(){
System.out.println("MyThread = " + threadLocal.get());
}
}
}
输出是:
main = 123
MyThread = null
那么这个时候怎么解决? InheritableThreadLocal 就可以解决这个问题。 先看一个官方对它的介绍:
/**
* This class extends ThreadLocal to provide inheritance of values
* from parent thread to child thread: when a child thread is created, the
* child receives initial values for all inheritable thread-local variables
* for which the parent has values. Normally the child's values will be
* identical to the parent's; however, the child's value can be made an
* arbitrary function of the parent's by overriding the childValue
* method in this class.
*
* Inheritable thread-local variables are used in preference to
* ordinary thread-local variables when the per-thread-attribute being
* maintained in the variable (e.g., User ID, Transaction ID) must be
* automatically transmitted to any child threads that are created.
*
* @author Josh Bloch and Doug Lea
* @see ThreadLocal
* @since 1.2
*/
也就是说,我们把上面的
public static ThreadLocal
改成
public static ThreadLocal
再运行,就会有结果:
main = 123
MyThread = 123
也就是子线程或者说新开的线程拿到了该值。那么,这个究竟是怎么实现的呢,key 都变了,为什么还可以拿到呢?
看下InheritableThreadLocal类的源码:
public class InheritableThreadLocal extends ThreadLocal {
/**
* 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.
*
* 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);
}
}
其实就是重写了3个方法。
首先,当我们调用 get 方法的时候,由于子类没有重写,所以我们调用了父类的 get 方法,看下父类get方法的源码
/**
* Returns the value in the current thread's copy of this
* thread-local variable. If the variable has no value for the
* current thread, it is first initialized to the value returned
* by an invocation of the {@link #initialValue} method.
*
* @return the current thread's value of this thread-local
*/
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
这里会有一个Thread.currentThread()
, getMap(t)
方法,所以就会得到这个线程 threadlocals。 但是,由于子类 InheritableThreadLocal 重写了 getMap()方法,再看上述代码,我们可以看到:
其实不是得到 threadlocals,而是得到 inheritableThreadLocals。 inheritableThreadLocals 之前一直没提及过,其实它也是 Thread 类的一个 ThreadLocalMap 类型的 属性,如下 Thread 类的部分代码:
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
/*
* InheritableThreadLocal values pertaining to this thread. This map is
* maintained by the InheritableThreadLocal class.
*/
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
那么,这里看 InheritableThreadLocal 重写的方法,感觉 inheritableThreadLocals 和 threadLocals 几乎是一模一样的作用,只是换了个名字而且,那么究竟 为什么在新的 线程中 通过 threadlocal.get()
方法还能得到值呢?
这时候要注意 childValue 方法,我们可以看下它的官方说明:
/**
* 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.
*
* 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
*/
这个时候,你明白了,是不是在 创建线程的时候做了手脚,做了一些值的传递,或者这里利用上了 inheritableThreadLocals 之类的。
其实,是的:
Thread thread = new MyThread();
这不是一个简简单单的 new 操作。当我们 new 一个 线程的时候:
/**
* Allocates a new {@code Thread} object. This constructor has the same
* effect as {@linkplain #Thread(ThreadGroup,Runnable,String) Thread}
* {@code (null, null, gname)}, where {@code gname} is a newly generated
* name. Automatically generated names are of the form
* {@code "Thread-"+}n, where n is an integer.
*/
public Thread() {
init(null, null, "Thread-" + nextThreadNum(), 0);
}
然后调用:
/**
* Initializes a Thread with the current AccessControlContext.
* @see #init(ThreadGroup,Runnable,String,long,AccessControlContext)
*/
private void init(ThreadGroup g, Runnable target, String name,
long stackSize) {
init(g, target, name, stackSize, null);
}
再调用:
/**
* 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
*/
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc) {
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);
if (parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
/* Stash the specified stack size in case the VM cares */
this.stackSize = stackSize;
/* Set thread ID */
tid = nextThreadID();
}
'ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);' 看这一句 :
源码跟进:
/**
* Factory method to create map of inherited thread locals.
* Designed to be called only from Thread constructor.
*
* @param parentMap the map associated with parent thread
* @return a map containing the parent's inheritable bindings
*/
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
return new ThreadLocalMap(parentMap);
}
继续跟进:
/**
* Construct a new map including all Inheritable ThreadLocals
* from given parent map. Called only by createInheritedMap.
*
* @param parentMap the map associated with parent thread.
*/
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
当我们创建一个新的线程的时候X,X线程就会有 ThreadLocalMap 类型的 inheritableThreadLocals ,因为它是 Thread 类的一个属性。
然后 先得到当前线程存储的这些值,例如 Entry[] parentTable = parentMap.table;
。再通过一个 for 循环,不断的把当前线程的这些值复制到我们新创建的线程X 的inheritableThreadLocals 中。就这样,就ok了。
那么这样会有一个什么结果呢?
结果就是我们创建的新线程X 的inheritableThreadLocals 变量中已经有了值了。那么我在新的线程X中调用threadlocal.get()
方法,首先会得到新线程X 的 inheritableThreadLocals,然后,再根据threadlocal.get()
中的 threadlocal,就能够得到这个值。
这样就避免了 新线程中得到的 threadlocals 没有东西。之前就是因为没有东西,所以才拿不到值。
所以说 整个 InheritableThreadLocal 的实现原理就是这样的。
threadlocal.get()
中的 threadlocal,就会得到值。