ThreadLocal是什么？

ThreadLocal从字面上的理解是本地线程的意思，然而事实上它是共享变量的一份拷贝，所以称之为ThreadLocalvVariable更合适

ThreadLocal对于多线程数据共享的意义？

下面举个多线程共享一个对象的例子（理解为单例）

package com.test.threadLocal;

public class Student {
    
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }   

}

package com.test.threadLocal;

public class MyThread extends Thread {
    
    private Student student;
    
    private String name;
    
    public MyThread(Student student,String name) {
        this.student = student;
        this.name = name;
    }

    @Override
    public void run() {
       this.student.setName(name);
       String myName = name;
       try
       {
         Thread.sleep(5000);
       }
       catch (InterruptedException e)
       {
         e.printStackTrace();
       }
       System.out.println("Thread: " + Thread.currentThread().getName()
                + ", name: " + student.getName() + ", myName: " + myName);
    }
}

package com.test.threadLocal;

public class Test
{
    public static void main(String[] args)
    {
        Student student = new Student();

        MyThread thread0 = new MyThread(student, "liubei");
        MyThread thread1 = new MyThread(student, "zhangfei");
        MyThread thread2 = new MyThread(student, "guanyu");
        
        thread0.start();
        thread1.start();
        thread2.start();
    }
}

运行结果

Thread: Thread-1, name: zhangfei, myName: zhangfei
Thread: Thread-2, name: zhangfei, myName: guanyu
Thread: Thread-0, name: zhangfei, myName: liubei

通过ThreadLocal改造Student代码，获取正确的结果

package com.test.threadLocal;

public class Student {
    
    private String name;
    
    private static ThreadLocal<String> threadLocal = new ThreadLocal<String>();

    public static String getName() {
        return threadLocal.get();
    }

    public static void setName(String name) {
        threadLocal.set(name);
    }   

}

运行结果

Thread: Thread-2, name: guanyu, myName: guanyu
Thread: Thread-1, name: zhangfei, myName: zhangfei
Thread: Thread-0, name: liubei, myName: liubei

ThreadLocal类中重要的方法

ThreadLocal类的set方法

    / ** Sets the current thread's copy of this thread-local variable
     * to the specified value.  Most subclasses will have no need to
     * override this method, relying solely on the {@link #initialValue}
     * method to set the values of thread-locals.
     *
     * @param value the value to be stored in the current thread's copy of
     *        this thread-local.
     */
    public void set(T value) {
        //获取当前线程
        Thread t = Thread.currentThread();
        //ThreadLocalMap是ThreadLocal的一个静态内部类，这里从数据结构上可以把ThreadLocalMap理解成HashMap，HashMap的解析会在之后的博客中解释
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

ThreadLocal类的getMap和createMap方法

ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }
void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

Thread类的threadLocals变量

/* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

ThreadLocal类的get方法

/**
     * 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();
    }

ThreadLocal的setInitialValue，initialValue方法

/**
     * Variant of set() to establish initialValue. Used instead
     * of set() in case user has overridden the set() method.
     *
     * @return the initial value
     */
    private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

/**
     * Returns the current thread's "initial value" for this
     * thread-local variable.  This method will be invoked the first
     * time a thread accesses the variable with the {@link #get}
     * method, unless the thread previously invoked the {@link #set}
     * method, in which case the {@code initialValue} method will not
     * be invoked for the thread.  Normally, this method is invoked at
     * most once per thread, but it may be invoked again in case of
     * subsequent invocations of {@link #remove} followed by {@link #get}.
     *
     * <p>This implementation simply returns {@code null}; if the
     * programmer desires thread-local variables to have an initial
     * value other than {@code null}, {@code ThreadLocal} must be
     * subclassed, and this method overridden.  Typically, an
     * anonymous inner class will be used.
     *
     * @return the initial value for this thread-local
     */
//用来设置初始化的value值，由子类决定是否重写
protected T initialValue() {
        return null;
    }

对ThreadLocal，ThreadLocalMap和Thread的关系的理解

简单的说每一个Thread对应的它的一个ThreadLocalMap,而ThreadLocalMap的key就是ThreadLocal，value是对应存储的值。从上述原代码中还可以得知，只要Thread没销毁，就能从ThreadLocalMap中get到你要的东西，换句话说，我们也可以将我们认为重要的信息存进去，其实Struts2中的ActionContext就是这么做的

synchronized和ThreadLocal的区别

对于多线程资源共享的问题，synchronized采用了“以时间换空间”的方式，而ThreadLocal采用了“以空间换时间”的方式。前者仅提供一份变量，让不同的线程排队访问，而后者为每一个线程都提供了一份变量，因此可以同时访问而互不影响。