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Starting and using background threads during class initialization can result in class initialization cycles and deadlock. For example, the main thread responsible for performing class initialization can block waiting for the background thread, which, in turn, will wait for the main thread to finish class initialization. This issue can arise, for example, when a database connection is established in a background thread during class initialization \[[Bloch 05b2005b|AA. Java References#Bloch 05b]\]. |
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public final class ConnectionFactory {
private static Connection dbConnection;
// Other fields ...
static {
Thread dbInitializerThread = new Thread(new Runnable() {
@Override public void run() {
// Initialize the database connection
try {
dbConnection = DriverManager.getConnection("connection string");
} catch (SQLException e) {
dbConnection = null;
}
}
});
// Other initialization, for example, start other threads
dbInitializerThread.start();
try {
dbInitializerThread.join();
} catch (InterruptedException ie) {
throw new AssertionError(ie);
}
}
public static Connection getConnection() {
if (dbConnection == null) {
throw new IllegalStateException("Error initializing connection");
}
return dbConnection;
}
public static void main(String[] args) {
// ...
Connection connection = getConnection();
}
}
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Statically initialized fields are guaranteed to be fully constructed before they are made visible to other threads. (seeSee guideline [CON28-J. Do not publish partially initialized objects|TSM03-J. Do not publish partially initialized objects] for more information). Consequently, the background thread must wait for the main (or foreground) thread to finish initialization before it can proceed. However, the {{ConnectionFactory}} 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 \[[Bloch 05b2005b|AA. Java References#Bloch 05b]\]. |
Similarly, it is inappropriate to start threads from constructors. (see See guideline TSM01-J. Do not let the ("this) " reference escape during object construction for more information). Creating timers that perform recurring tasks and starting those timers from within code responsible for initialization also creates introduces liveness issues.
Compliant Solution (static Initializer, No Background Threads)
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public final class ConnectionFactory {
private static final ThreadLocal<Connection> connectionHolder
= new ThreadLocal<Connection>() {
@Override public Connection initialValue() {
try {
Connection dbConnection = DriverManager.getConnection("connection string");
return dbConnection;
} catch (SQLException e) {
return null;
}
}
};
// Other fields ...
static {
// Other initialization (do not start any threads)
}
public static Connection getConnection() {
Connection connection = connectionHolder.get();
if (connection == null) {
throw new IllegalStateException("Error initializing connection");
}
return connection;
}
public static void main(String[] args) {
// ...
Connection connection = getConnection();
}
}
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<ac:structured-macro ac:name="anchor" ac:schema-version="1" ac:macro-id="c406b7d75dafca84-f1282342-4a2641dc-a318afd7-0c6464f0acc61c6371546efc"><ac:parameter ac:name="">CON20-EX1</ac:parameter></ac:structured-macro> *CON20TSM02-EX1:* It is permissible to start a background thread during class initialization provided the thread does not access any fields. For example, the {{ObjectPreserver}} class (based on \[[Patterns 022002|AA. Java References#Patterns 02]\]) shown below provides a mechanism for storing object references, which prevents an object from being garbage-collected, even if the object is not dereferencedde-referenced in the future. |
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public final class ObjectPreserver implements Runnable {
private static ObjectPreserver lifeLine = new ObjectPreserver();
private ObjectPreserver() {
Thread thread = new Thread(this);
thread.setDaemon(true);
thread.start(); // Keep this object alive
}
// Neither this class nor HashMap will be garbage-collected.
// References from HashMap to other objects will also exhibit this property
private static final ConcurrentHashMap<Integer,Object> protectedMap
= new ConcurrentHashMap<Integer,Object>();
public synchronized void run() {
try {
wait();
} catch (InterruptedException e) {
Thread.currentThread().interrupt(); // Reset interrupted status
}
}
// Objects passed to this method will be preserved until
// the unpreserveObject method is called
public static void preserveObject(Object obj) {
protectedMap.put(0, obj);
}
// Returns the same instance every time
public static Object getObject() {
return protectedMap.get(0);
}
// Unprotect the objects so that they can be garbage-collected
public static void unpreserveObject() {
protectedMap.remove(0);
}
}
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This is a singleton class. (See guideline MSC16-J. Address the shortcomings of the Singleton design pattern for more information on how to defensively code singleton classes.) The initialization involves creating a background thread using the current instance of the class. The thread waits indefinitely by invoking Object.wait(). Consequently, this object exists persists for the remainder of the Java Virtual Machine's ( JVM's ) lifetime. Because the object is managed by a daemon thread, the thread does not hinder a normal shutdown of the JVM.
While the initialization does involve a background thread, the that thread does not access any fields or create any liveness or safety issues. Consequently, this code is a safe and useful exception to this guideline.
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Starting and using background threads during class initialization can result in deadlock conditions.
Rule Guideline | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
CON20 TSM02- J | low | likely probable | high | P3 P2 | L3 |
Related Vulnerabilities
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References
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\[[Bloch 05b2005b|AA. Java References#Bloch 05b]\] 8. "Lazy Initialization" \[[Patterns 022002|AA. Java References#Patterns 02]\] Chapter 5, Creational Patterns, Singleton |
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