ThreadLocal

ThreadLocal : 线程局部变量

A: Thread-local variables .these variable differ from their  normal counterparts  in that each thread  accesses one (via set or get method) has its own, independently initialized copy of the variable.


B:Each thread hold an  implicit reference to its copy of a thread-local variables as long as the thread is alive. If a thread goes away, all of its copies of thread-local variables are subject to garbage collection



C:case study:

public class SerialNum
{
// The next serial number to be assigned
    private static int nextSerialNum = 0;
    private static ThreadLocal serialNum = new ThreadLocal()
    {
        protected synchronized Object initialValue()
        {
            return new Integer(nextSerialNum++);
        }
    };
    public static int get()
    {
        return ((Integer) (serialNum.get())).intValue();
    }
}

The private static  thread-local instances maintains “serial num” for each thread that invokes the class’s static SerialNum.get() method which returns the current thread serial num.( a thread serial serial num is assigned the first time  it invokes SerialNum.get() method and  remains unchanged on subsequent calls)

C2:

public class ConnectionPool
{
    private static ThreadLocal<Connection> local = new ThreadLocal<Connection>();
   public static Connection getConnection()
    {
        Connection conn = local.get();
        if (conn == null) {
            conn = new Connection();
            local.set(conn);
        }
        return conn;
    }
}

The private static ThreadLocal maintains Connection objects for each thread that invokes ConnectionPool.getConnection() Method which return connection object of current thread.
   A thread invoke ThreadLocal.get() method to get a connection object . for the first time  the connection object is null , it will be assigned a new connection object and set by invoking local.set() method and remains unchanged on subsequent calls.



D: How it works ?

 

• ThreadLocalMap is maintained by ThreadLocal and attached to  each  Thread.
• When ThreadLocal.set() method is invoked  by each thread

      1.ThreadLocal will get current thread’s ThreadLocalMap
      2.If ThreadLocalMap is null then createMap for the current thread.
      3. Put the value into the ThreadLocalMap. key(ThreadLocal) is the main refer fields  for ThreadLocalMap.

• When ThreadLocal.get() method is invoked by each thread:
  
    1.ThreadLocal will get current thread’s ThreadLocalMap
    2. If ThreadLocalMap is  not null then get value from the ThreadLocalMap
    3. If ThreadLocalMap is null, it will invoke initialValue and createMap method.

• In order to implement the copy of value(unchanged on subsequent calls for every thread),ThreadLocal  instances are  typically  private static.