一、ThreadLocal的作用

ThreadLocal以線程作為作用域，針對每個線程維護他們自己的數(shù)據(jù)。每個線程使用同一個ThreadLocal存儲數(shù)據(jù)，但是拿到的數(shù)據(jù)都只會是當(dāng)前這個線程所存儲的數(shù)據(jù)，不會拿到其他線程存儲的數(shù)據(jù)。

例子來自

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.獲取當(dāng)前線程對象
2.獲取當(dāng)前線程維護的Values對象
3.如果values是空，為當(dāng)前線程創(chuàng)建一個Values對象
4.將ThreadLocal引用和值傳遞給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對象
每個Thread對象都有一個Values對象，這個Values對象的類型是ThreadLocal.Values。它的put方法

1.遍歷數(shù)組，如果當(dāng)前ThreadLocal已經(jīng)維護了一個對象，那么替換這個對象為新值
2.如果沒有存過數(shù)據(jù)，那么找數(shù)組中的一個空白位置，在index和index+1這兩個相鄰的位置分別存儲鍵和值

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.獲取當(dāng)前線程的Values對象
2.獲取Values內(nèi)部維護的數(shù)組
3 .計算出ThreadLocal對象的index
4.如果index在數(shù)組中存在，那么從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、總結(jié)

1.每一個Thread內(nèi)部都持有一個ThreadLocal.Values的靜態(tài)對象
2.ThreadLocal.Values內(nèi)部維護了一個數(shù)組存儲鍵和值
3.ThreadLocal拿到當(dāng)前Thread所維護的Values對象，并將自身的引用和要存的值傳遞個Values
4.Values根據(jù)threadlocal引用計算出它的index，如果在index位置有值存在就替換原來的值，如果不存在就替換原來的值