ThreadLocal基本知識點分析

一. ThreadLocal是什麼？

ThreadLocal是一個將在多線程中爲每一個線程創建單獨的變量副本的類; 當使用ThreadLocal來維護變量時, ThreadLocal會爲每個線程創建單獨的變量副本, 避免因多線程操作共享變量而導致的數據不一致的情況。

二. ThreadLocal使用方法

1、將需要被多線程訪問的屬性使用ThreadLocal變量來定義;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class DBConnectionFactory {

    private static final ThreadLocal<Connection> dbConnectionLocal = new ThreadLocal<Connection>() {
        @Override
        protected Connection initialValue() {
            try {
                return DriverManager.getConnection("", "", "");
            } catch (SQLException e) {
                e.printStackTrace();
            }
            return null;
        }
    };

    public Connection getConnection() {
        return dbConnectionLocal.get();
    }
}

這樣在Client獲取Connection的時候, 每個線程獲取到的Connection都是該線程獨有的, 做到Connection的線程隔離; 所以並不存在線程安全問題。

三. 分析set(),get()方法

public void set(T value) {
		//獲取當前的線程
        Thread t = Thread.currentThread();
        //獲取到當前線程的變量threadLocals，在Thread中定義， ThreadLocal.ThreadLocalMap threadLocals = null;
        ThreadLocalMap map = getMap(t);
     //如果當前線程的threadLocals是否初始化
        if (map != null)
        //已經初始化(即不爲null)但是不存在以當前ThreadLocal對象爲Key的的對象,
            map.set(this, value);
        else
            createMap(t, value);
    }

ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
 }
 //set設置值
 private void set(ThreadLocal<?> key, Object value) {

            // We don't use a fast path as with get() because it is at
            // least as common to use set() to create new entries as
            // it is to replace existing ones, in which case, a fast
            // path would fail more often than not.

            Entry[] tab = table;
            int len = tab.length;
            //計算索引值
            int i = key.threadLocalHashCode & (len-1);

            for (Entry e = tab[i];e != null;e = tab[i = nextIndex(i, len)]){
                ThreadLocal<?> k = e.get();
                //如果當前線程的threadLocals已經初始化(即不爲null) 並且存在以當前
                //ThreadLocal對象爲Key的值, 則直接返回當前線程要獲取的對象(本例中爲Connection);
                if (k == key) {
                    e.value = value;
                    return;
                }
	//如果當前線程的threadLocals已經初始化(即不爲null)但是不存在以當前ThreadLocal對象爲Key的的對象, 
	//那麼重新創建一個Connection對象, 並且添加到當前線程的threadLocals Map中,並返回
                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }

            tab[i] = new Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }
 //初始化threadLocals
void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

1、set()方法的步驟：

1. 首先獲取當前線程對象t, 然後從線程t中獲取到ThreadLocalMap的成員屬性threadLocals
2. 如果當前線程的threadLocals已經初始化(即不爲null) 並且存在以當前ThreadLocal對象爲Key的值, 則直接返回當前線程要獲取的對象(本例中爲Connection);
3. 如果當前線程的threadLocals已經初始化(即不爲null)但是不存在以當前ThreadLocal對象爲Key的的對象, 那麼重新創建一個Connection對象, 並且添加到當前線程的threadLocals Map中,並返回
4. 如果當前線程的threadLocals屬性還沒有被初始化, 則重新創建一個ThreadLocalMap對象, 並且創建一個Connection對象並添加到ThreadLocalMap對象中並返回。

ThreadLocal轉載，這個寫的很好