线程之上下文传递inheritableThreadLocal

inheritableThreadLocal

能满足上下文之间的数据传递

前提是在父类线程中创建子类线程;

 

参见源码如下:

Thread类有两个属性

ThreadLocal.ThreadLocalMap threadLocals = null;

ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

 

我们在进行new Thread()会调用下方init方法:

该方法会获取父线程的inheritableThreadLocals并赋值到当前线程

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);
    if (inheritThreadLocals && 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();
}

其中springcloud的hystrix采用线程池进行资源隔离

此时可以采用HystrixConcurrencyStrategy进行上下文传递，为什么不采用inheritableThreadLocal进行上下文传递

这里就要说到inheritableThreadLocal传递的特性，创建线程

hystrix采用的是线程池技术，所以不是每次请求都一定会创建线程，如果是获取线程资源池的线程而不是创建线程

则父线程的属性并不会被传递给子线程