当前线程的值传递，ThreadLocal
ThreadLocal，顾名思义，即线程本地环境。
具体用途是，在本线程上下文内，通过ThreadLocal.set()设值，可通过ThreadLocal.get()取值。
public class BasicUsage {
private static ThreadLocal<Integer> threadLocal = new ThreadLocal<Integer>();
public static void main(String[] args) {
threadLocal.set(1);
otherMethod();
}
public static void otherMethod() {
System.out.println("threadLocal.get() -> " + threadLocal.get()); // 其它Class、其它方法，只要在此线程内就可获取
}
}结果：threadLocal.get() -> 1
ThreadLocal源码浅读
如何设置值
这是设置值的方法，可以看到，关键在于将值存放于ThreadLocalMap中。
而ThreadLocalMap的功能则相当于一个key-value的容器，key是ThreadLocal类型，value是Object类型。
public void set(T value) {
Thread t = Thread.currentThread(); // 获取当前线程
ThreadLocalMap map = getMap(t); // 根据线程对象获取ThreadLocalMap
if (map != null)
map.set(this, value); // 将值设置进ThreadLocalMap中
else
createMap(t, value); // 创建ThreadLocalMap
}跟踪进去getMap(t)，可知ThreadLocalMap是声明在Thread中的threadLocals变量中
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}跟踪进去createMap(t, value)，可以看到实例化ThreadLocalMap的代码，我们留意到第一个参数是this，也就是ThreadLocal对象，下文会有提到：
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}跟踪进ThreadLocalMap的构造方法，可以发现它内部维护一个Entry数组，构造方法的代码基本围绕将这个值应存放于数组的哪个下标。这里请注意下实例化Entry的参数，继续跟踪Entry：
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY]; // 初始化初始长度的Entry数组
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1); // 与数组长度做与运算，得到存放的下标
table[i] = new Entry(firstKey, firstValue); // 实例化一个Entry对象
size = 1; // 记录当前的大小
setThreshold(INITIAL_CAPACITY); // 设置阀值，未以后扩容作计算依据
}跟踪进Entry的构造方法，发现Entry继承WeakReference，以ThreadLocal作为Key，自己存储value：
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */ // 此值与threadLocal关联
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}整体结构
父线程、子线程中的值传递
JDK的InheritableThreadLocal
使用ThreadLocal中如果使用多线程，会发现父线程设置的值在子线程中无法获取，JDK中有InheritableThreadLocal解决此问题。
public class SubThreadUsage {
private static ThreadLocal<Integer> threadLocal = new InheritableThreadLocal<Integer>();
public static void main(String[] args) {
threadLocal.set(1);
// 新启一个线程
new Thread(new Runnable() {
@Override
public void run() {
otherMethod();
}
}).start();
}
public static void otherMethod() {
System.out.println("threadLocal.get() -> " + threadLocal.get());
}
}结果：threadLocal.get() -> 1
原理简述：
- 看
InheritableThreadLocal，会发现其继承ThreadLocal<T>，并且数据存放在Thread的变量inheritableThreadLocals中，变量类型是ThreadLocal.ThreadLocalMap - 在
Thread构造方法调用的init()中，可看见如果parent.inheritableThreadLocals不为空，则ThreadLocal.createInheritedMap()拷贝ThreadLocalMap，拷贝实际调用的是构造方法ThreadLocalMap(ThreadLocalMap)，为浅拷贝
所以，如果运用线程池等线程复用技术，传递的数据会有遗留：
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
public class SubThreadReuseThreadUsage {
private static ThreadLocal<Integer> threadLocal = new InheritableThreadLocal<Integer>();
public static void main(String[] args) throws InterruptedException {
threadLocal.set(1);
/* 声明多线程组件 */
ExecutorService executorService = Executors.newSingleThreadExecutor();
Runnable runnableA = new Runnable() {
@Override
public void run() {
otherMethodA();
}
};
Runnable runnableB = new Runnable() {
@Override
public void run() {
otherMethodB();
}
};
// 运行一个线程
executorService.execute(runnableA);
TimeUnit.SECONDS.sleep(1); // 睡眠，让上面线程跑完
/* 运行一个线程 */
executorService.execute(runnableB);
}
public static void otherMethodA() {
System.out.println("threadLocal.get() -> " + threadLocal.get());
threadLocal.set(2);
}
public static void otherMethodB() {
System.out.println("threadLocal.get() -> " + threadLocal.get());
}
}结果：
threadLocal.get() -> 1
threadLocal.get() -> 2TransmittableThreadLocal
而线程复用技术因减低线程开销而常用，所以需解决此问题，阿里开源的TransmittableThreadLocal是一个方案，其实现加强了InheritableThreadLocal。
用TransmittableThreadLocal、TtlRunnable的简单例子：
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import com.alibaba.ttl.TransmittableThreadLocal;
import com.alibaba.ttl.TtlRunnable;
public class SubThreadReuseThreadUsage {
private static ThreadLocal<Integer> threadLocal = new TransmittableThreadLocal<Integer>();
public static void main(String[] args) throws InterruptedException {
threadLocal.set(1);
/* 声明多线程组件 */
ExecutorService executorService = Executors.newSingleThreadExecutor();
Runnable runnableA = new Runnable() {
@Override
public void run() {
otherMethodA();
}
};
Runnable runnableB = new Runnable() {
@Override
public void run() {
otherMethodB();
}
};
TtlRunnable ttlRunnableA = TtlRunnable.get(runnableA);
TtlRunnable ttlRunnableB = TtlRunnable.get(runnableB);
// 运行一个线程
executorService.execute(ttlRunnableA);
TimeUnit.SECONDS.sleep(1); // 睡眠，让上面线程跑完
/* 运行一个线程 */
executorService.execute(ttlRunnableB);
}
public static void otherMethodA() {
System.out.println("threadLocal.get() -> " + threadLocal.get());
threadLocal.set(2);
}
public static void otherMethodB() {
System.out.println("threadLocal.get() -> " + threadLocal.get());
}
}结果：
threadLocal.get() -> 1
threadLocal.get() -> 1这里通过使用TtlRunnable和TtlCallable完成，还可以通过使用TtlExecutors完成，另外还有无侵入方案Java Agent，详情。