...
| Wiki Markup |
|---|
The code in the {{static}} block is responsible for initialization, and starts a background thread. The background thread attempts to assign to the {{flag}} but needs to wait before initialization of the {{Lazy}} class has finished. Remember that statically-initialized fields are guaranteed to be fully constructed before becoming visible to other threads (see [CON26-J. Do not publish partially initialized objects] for more info). Consequently the background thread must wait for the foreground thread to finish initialization before it may proceed. However, the {{Lazy}} class's main thread invokes the {{join()}} method which waits for the background thread to finish. This interdependency causes a class initialization cycle that results in a deadlock situation. \[[Bloch 05b|AA. Java References#Bloch 05b]\] |
flag is not required to be volatile because the initialization is being carried out in a static initializer and is guaranteed to finish before the object becomes usable. Similar to this noncompliant code example, threads should not be started from constructors. See CON14-J. Do not let the "this" reference escape during object construction for more information.
...
| Code Block | ||
|---|---|---|
| ||
public class Lazy {
private static boolean flag = false;
private static final ThreadLocal<Connection> connectionHolder
= new ThreadLocal<Connection>() {
public Connection initialValue() {
try {
Connection conn = DriverManager.getConnection("connectionstring");
flag = true;
return conn;
} catch (SQLException e) {
return null;
}
}
}
};
public static Connection getConnection() {
return connectionHolder.get();
}
public static void main(String[] args) {
Connection conn = getConnection();
System.out.println(flag);
}
}
|
...