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This noncompliant code example begins initializing the class Lazy. The code in the static block is responsible for initialization, and starts a background thread. The background thread attempts to initialize a database connection but needs to wait until initialization of all members of the Lazy class, including dbConnection, has finished.
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public final class Lazy {
private static Connection dbConnection;
static {
Thread dbInitializerThread = new Thread(new Runnable() {
public void run() {
// Initialize the database connection
try {
dbConnection = DriverManager.getConnection("connectionstring");
} catch (SQLException e) {
dbConnection = null;
}
}
});
dbInitializerThread.start();
try {
dbInitializerThread.join();
} catch(InterruptedException ie) {
throw new AssertionError(ie);
}
// Other initialization
}
public static Connection getConnection() {
if(dbConnection == null) {
throw new IllegalStateException("Error initializing connection");
}
return dbConnection;
}
public static void main(String[] args) {
// ...
Connection connection = Lazy.getConnection();
}
}
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| Wiki Markup |
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Recall 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fields are guaranteed to be fully constructed before becoming visible to other threads (see [CON26-J. Do not publish partially initialized objects] for more information). Consequently, the background thread must wait for the foreground thread to finish initialization before it can 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. \[[Bloch 05b|AA. Java References#Bloch 05b]\] |
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