ThreadLocal理解

一、ThreadLocal的作用

ThreadLocal以线程作为作用域，针对每个线程维护他们自己的数据。每个线程使用同一个ThreadLocal存储数据，但是拿到的数据都只会是当前这个线程所存储的数据，不会拿到其他线程存储的数据。

例子来自

public class MainActivity extends AppCompatActivity {
    private static final String TAG = "ThreadLoacalTest";
    private ThreadLocal<Boolean> mBooleanThreadLocal = new ThreadLocal<>();
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        init();
    }

    private void init(){
        mBooleanThreadLocal.set(true);
        printThreadLocal();

        new Thread("Thread#1"){
            @Override
            public void run() {
                mBooleanThreadLocal.set("name");
                printThreadLocal();
            }
        }.start();

        new Thread("Thread#2"){
            @Override
            public void run() {
                printThreadLocal();
            }
        }.start();
    }

    private void printThreadLocal() {
        Log.d(TAG, "[Thread#" + Thread.currentThread().getName() + "#" + Thread.currentThread().getId() + "]mBooleanThreadLocal=" + mBooleanThreadLocal.get() );
    }
}
07-17 11:23:19.773 3162-3162/com.chuckchan.threadlocalsample D/ThreadLoacalTest: [Thread#main#2]mBooleanThreadLocal=true
07-17 11:23:19.777 3162-3181/com.chuckchan.threadlocalsample D/ThreadLoacalTest: [Thread#Thread#2#151]mBooleanThreadLocal=null
07-17 11:23:19.798 3162-3180/com.chuckchan.threadlocalsample D/ThreadLoacalTest: [Thread#Thread#1#150]mBooleanThreadLocal=false

二、 ThreadLocal原理

1.ThreadLocal的set方法

1.获取当前线程对象
2.获取当前线程维护的Values对象
3.如果values是空，为当前线程创建一个Values对象
4.将ThreadLocal引用和值传递给Values

    public void set(T value) {
        Thread currentThread = Thread.currentThread();
        Values values = values(currentThread);
        if (values == null) {
            values = initializeValues(currentThread);
        }
        values.put(this, value);
    }

    Values values(Thread current) {
        return current.localValues;
    }

2.Thread的Values对象
每个Thread对象都有一个Values对象，这个Values对象的类型是ThreadLocal.Values。它的put方法

1.遍历数组，如果当前ThreadLocal已经维护了一个对象，那么替换这个对象为新值
2.如果没有存过数据，那么找数组中的一个空白位置，在index和index+1这两个相邻的位置分别存储键和值

void put(ThreadLocal<?> key, Object value) {
    cleanUp();

    // Keep track of first tombstone. That's where we want to go back
    // and add an entry if necessary.
    int firstTombstone = -1;

    for (int index = key.hash & mask;; index = next(index)) {
        Object k = table[index];

        if (k == key.reference) {
            // Replace existing entry.
            table[index + 1] = value;
            return;
        }

        if (k == null) {
            if (firstTombstone == -1) {
                // Fill in null slot.
                table[index] = key.reference;
                table[index + 1] = value;
                size++;
                return;
            }

            // Go back and replace first tombstone.
            table[firstTombstone] = key.reference;
            table[firstTombstone + 1] = value;
            tombstones--;
            size++;
            return;
        }

        // Remember first tombstone.
        if (firstTombstone == -1 && k == TOMBSTONE) {
            firstTombstone = index;
        }
    }
}

ThreadLocal的get方法

1.获取当前线程的Values对象
2.获取Values内部维护的数组
3 .计算出ThreadLocal对象的index
4.如果index在数组中存在，那么从index+1的位置取出值

  public T get() {
        // Optimized for the fast path.
        Thread currentThread = Thread.currentThread();
        Values values = values(currentThread);
        if (values != null) {
            Object[] table = values.table;
            int index = hash & values.mask;
            if (this.reference == table[index]) {
                return (T) table[index + 1];
            }
        } else {
            values = initializeValues(currentThread);
        }

        return (T) values.getAfterMiss(this);
    }

三、总结

1.每一个Thread内部都持有一个ThreadLocal.Values的静态对象
2.ThreadLocal.Values内部维护了一个数组存储键和值
3.ThreadLocal拿到当前Thread所维护的Values对象，并将自身的引用和要存的值传递个Values
4.Values根据threadlocal引用计算出它的index，如果在index位置有值存在就替换原来的值，如果不存在就替换原来的值