ThreadLocal用法
Java中线程的同步机制保证了多线程访问共享变量的安全性，通常我们使用synchronized关键字来实现。在多个线程对共享变量进行读写操作时，同步锁保证了同一时间只有一个线程对共享变量进行操作，概括地说，这是一种“以时间换空间”的解决策略。
在JDK1.2中引入了ThreadLocal类来提供了一种“以空间换时间”的同步解决策略。ThreadLocal内部维护了一份类似Map的静态变量ThreadLocalMap，其中key为当前线程，value为共享变量。JDK1.5引入泛型，ThreadLocal也同时支持泛型。
其具体实现如下
public class ThreadLocal<T> {
/**
* ThreadLocals rely on per-thread hash maps attached to each thread
* (Thread.threadLocals and inheritableThreadLocals). The ThreadLocal
* objects act as keys, searched via threadLocalHashCode. This is a
* custom hash code (useful only within ThreadLocalMaps) that eliminates
* collisions in the common case where consecutively constructed
* ThreadLocals are used by the same threads, while remaining well-behaved
* in less common cases.
*/
private final int threadLocalHashCode = nextHashCode();
/**
* The next hash code to be given out. Accessed only by like-named method.
*/
private static int nextHashCode = 0;
/**
* The difference between successively generated hash codes - turns
* implicit sequential thread-local IDs into near-optimally spread
* multiplicative hash values for power-of-two-sized tables.
*/
private static final int HASH_INCREMENT = 0x61c88647;
/**
* Compute the next hash code. The static synchronization used here
* should not be a performance bottleneck. When ThreadLocals are
* generated in different threads at a fast enough rate to regularly
* contend on this lock, memory contention is by far a more serious
* problem than lock contention.
*/
private static synchronized int nextHashCode() {
int h = nextHashCode;
nextHashCode = h + HASH_INCREMENT;
return h;
}
/**
* Creates a thread local variable.
*/
public ThreadLocal() {
}
/**
* Returns the value in the current thread's copy of this thread-local
* variable. Creates and initializes the copy if this is the first time
* the thread has called this 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)
return (T)map.get(this);
// Maps are constructed lazily. if the map for this thread
// doesn't exist, create it, with this ThreadLocal and its
// initial value as its only entry.
T value = initialValue();
createMap(t, value);
return value;
}
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Many applications will have no need for
* this functionality, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current threads' 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);
}
/**
* Get the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @return the map
*/
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
/**
* Create the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @param firstValue value for the initial entry of the map
* @param map the map to store.
*/
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
.......
/**
* ThreadLocalMap is a customized hash map suitable only for
* maintaining thread local values. No operations are exported
* outside of the ThreadLocal class. The class is package private to
* allow declaration of fields in class Thread. To help deal with
* very large and long-lived usages, the hash table entries use
* WeakReferences for keys. However, since reference queues are not
* used, stale entries are guaranteed to be removed only when
* the table starts running out of space.
*/
static class ThreadLocalMap {
........
}
}
从中很清晰的可以看出，多个线程拥有自己一份单独的ThreadLocalMap，共享变量对于每个线程都是单独的一份，因此不会造成线程的安全问题。
JDBC的ConnectionManager类就是以这种方式来实现数据库连接Connection对象线程隔离。
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class ConnectionManager {
private static ThreadLocal<Connection> connectionHolder = new ThreadLocal<Connection>() {
@Override
protected Connection initialValue() {
Connection conn = null;
try {
conn = DriverManager.getConnection(
"jdbc:mysql://localhost:3306/test", "username",
"password");
} catch (SQLException e) {
e.printStackTrace();
}
return conn;
}
};
public static Connection getConnection() {
return connectionHolder.get();
}
public static void setConnection(Connection conn) {
connectionHolder.set(conn);
}
}
但是，有些情况ThreadLocal可能并不适用，例如存储大量数据的共享变量，或共享变量只能被创建一次时，就只能通过synchronized来实现了。
网友评论