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ThreadLocal (三）：为何TransmittableThreadLocal
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一、示例
线程池内的线程并没有父子关系，所以不适合InheritableThreadLocal的使用场景
public class ThreadPoolInheritableThreadLocalDemo {
// static ThreadLocal<String> threadLocal = new InheritableThreadLocal<>();
// static ExecutorService pool = Executors.newFixedThreadPool(2);
static TransmittableThreadLocal<String> threadLocal = new TransmittableThreadLocal<>();
static ExecutorService pool = TtlExecutors.getTtlExecutorService(Executors.newFixedThreadPool(3));
public static void main(String[] args) {
for(int i=0;i<100;i++) {
int j = i;
pool.execute(new Thread(new Runnable() {
@Override
public void run() {
ThreadPoolInheritableThreadLocalDemo.threadLocal.set("superWorld"+j);
ThreadPoolInheritableThreadLocalDemo.pool.execute(new Runnable() {
@Override
public void run() {
System.out.println(Thread.currentThread().getName() +
" : " +
ThreadPoolInheritableThreadLocalDemo.threadLocal.get());
}
});
}
}));
}
}
}
二、TransmittableThreadLocal实现分析
读取线程间传递的ThreadLocal 值比较麻烦，ThreadLocal 和 InheritableThreadLocal 都没有开放内部的 ThreadLocalMap，不能直接读取。
所以要么自己完全实现一套 ThreadLocalMap 机制(如 Netty 的 FastThreadLocal），要么就是自己实现 ThreadLocal 的子类，在每次调用 ThreadLocal
的 set/get/remove 等接口的时候，为 Thread 记录到底绑定了哪些需要发生线程间传递的 ThreadLocal 对象。
/**
*实际存储值的工作还是父类ThreadLocal完成
*TransmittableThreadLocal 只是记录了哪些线程使用了TransmittableThreadLocal对象
*/
@Override
public final void set(T value) {
super.set(value);
if (null == value) { // may set null to remove value
removeValue();
} else {
addValue();
}
}/**
*holder 只是为了记录使用了哪些 TransmittableThreadLocal 对象
*在构造TtlRunnable/TtlCallable 的时候, 通过holder取得对应的TransmittableThreadLocal
*InheritableThreadLocal的默认值是WeakHashMap
*/
private static InheritableThreadLocal<Map<TransmittableThreadLocal<?>, ?>> holder =
new InheritableThreadLocal<Map<TransmittableThreadLocal<?>, ?>>() {
@Override
protected Map<TransmittableThreadLocal<?>, ?> initialValue() {
return new WeakHashMap<TransmittableThreadLocal<?>, Object>();
} @Override
protected Map<TransmittableThreadLocal<?>, ?> childValue(Map<TransmittableThreadLocal<?>, ?> parentValue) {
return new WeakHashMap<TransmittableThreadLocal<?>, Object>(parentValue);
}
}; private void addValue() {
if (!holder.get().containsKey(this)) {
holder.get().put(this, null); // WeakHashMap supports null value.
}
} private void removeValue() {
holder.get().remove(this);
}
调用ThreadPoolInheritableThreadLocalDemo.threadLocal.set("superWorld"+j)时，
holder.get().containskey(this) 为false
2. TtlRunnable
构造TtlRunable时，设置线程对应的Map<TransmittableThreadLocal<?>, Object>>
private TtlRunnable(Runnable runnable, boolean releaseTtlValueReferenceAfterRun) {
//
this.copiedRef = new AtomicReference<Map<TransmittableThreadLocal<?>, Object>>(TransmittableThreadLocal.copy());
this.runnable = runnable;
this.releaseTtlValueReferenceAfterRun = releaseTtlValueReferenceAfterRun;
}
TransmittableThreadLocal.copy
static Map<TransmittableThreadLocal<?>, Object> copy() {
Map<TransmittableThreadLocal<?>, Object> copy = new HashMap<TransmittableThreadLocal<?>, Object>();
for (TransmittableThreadLocal<?> threadLocal : holder.get().keySet()) {
copy.put(threadLocal, threadLocal.copyValue());
}
return copy;
}
3.运行时，备份和恢复Map<TransmittableThreadLocal<?>, Object>
TtlRunnable#run
@Override
public void run() {
Map<TransmittableThreadLocal<?>, Object> copied = copiedRef.get();
if (copied == null || releaseTtlValueReferenceAfterRun && !copiedRef.compareAndSet(copied, null)) {
throw new IllegalStateException("TTL value reference is released after run!");
} Map<TransmittableThreadLocal<?>, Object> backup = TransmittableThreadLocal.backupAndSetToCopied(copied);
try {
runnable.run();
} finally {
TransmittableThreadLocal.restoreBackup(backup);
}
}
参考：
transmittableThreadLocal
