一个ThreadLocal的使用场景

深碍√TFBOYSˉ_ 2022-05-14 05:59 258阅读 0赞

/**
    * 有了驱动类后，通过驱动类创建连接
    * @return
    */
    private static ThreadLocal<Connection> tol =new ThreadLocal<Connection>();
    public static Connection getConnection() {
    	//声明连接
    	try {
    	Connection conn=tol.get();
    		
    	    if (conn==null) {
    		conn=DriverManager.getConnection(
    		    PROPERTIES.getProperty("jdbc.url"), 
    		    PROPERTIES.getProperty("jdbc.username"),
    		    PROPERTIES.getProperty("jdbc.password") 
    		);
    	    tol.set(conn);
    	    }
    	    return conn;
            } catch (Exception e) {
    	    e.printStackTrace();
    	    throw new RuntimeException("获取连接失败!~");
    	}
    }

每次其他类通过**getConnection（）**方法获取连接时，都会先去本地线程变量中获取。

若获取不到，获取的结果为空。

则新创建连接，这样保证了不会让各层都创建不同的连接，不方便业务控制。

第一次获取的连接被ThreadLocal的set方法存储在本地线程变量中。

## **Thread**的**get**方法 ##

**从本线程中获取其绑定的资源**

/**
         * 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();
        }

## **Thread**的**set**方法 ##

**主要用于本线程与所需绑定资源的绑定**

/**
         * 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) {
            Thread t = Thread.currentThread();
            ThreadLocalMap map = getMap(t);
            if (map != null)
                map.set(this, value);
            else
                createMap(t, value);
        }

## **Thread**的**remove**方法 ##

**主要用于本线程与所绑定资源的解绑**（**并不会销毁资源！**）

/**
         * Removes the current thread's value for this thread-local
         * variable.  If this thread-local variable is subsequently
         * {@linkplain #get read} by the current thread, its value will be
         * reinitialized by invoking its {@link #initialValue} method,
         * unless its value is {@linkplain #set set} by the current thread
         * in the interim.  This may result in multiple invocations of the
         * {@code initialValue} method in the current thread.
         *
         * @since 1.5
         */
         public void remove() {
             ThreadLocalMap m = getMap(Thread.currentThread());
             if (m != null)
                 m.remove(this);
         }