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 2005b]. Consequently, programs must ensure that class initialization is complete before starting any threads.
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In this noncompliant code example, the static initializer starts a background thread as part of class initialization. The background thread attempts to initialize a database connection but should wait until all members of the ConnectionFactory class, including dbConnection, are initialized.
| Code Block | ||
|---|---|---|
| ||
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fields are guaranteed to be fully constructed before they are made visible to other threads (see rule TSM03-J. Do not publish partially initialized objects for more information).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 2005b].
Similarly, it is inappropriate to start threads from constructors (see rule 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 introduces liveness issues.
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This compliant solution initializes all fields on the main thread rather than spawning background threads from the static initializer.
| Code Block | ||
|---|---|---|
| ||
public final class ConnectionFactory {
private static Connection dbConnection;
// Other fields ...
static {
// Initialize a database connection
try {
dbConnection = DriverManager.getConnection("connection string");
} catch (SQLException e) {
dbConnection = null;
}
// Other initialization (do not start any threads)
}
// ...
}
|
...
This compliant solution initializes the database connection from a ThreadLocal object so that each thread can obtain its own unique instance of the connection.
| Code Block | ||
|---|---|---|
| ||
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();
}
}
|
...
| Anchor | ||||
|---|---|---|---|---|
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TSM02-J-EX0: Programs are permitted to start a background thread (or threads) during class initialization, provided the thread cannot access any fields. For example, the following
ObjectPreserver class (based on [Grand 2002]) provides a mechanism for storing object references, which prevents an object from being garbage-collected even when the object is never again dereferenced.| Code Block | ||
|---|---|---|
| ||
public final class ObjectPreserver implements Runnable {
private static final 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 rule MSC07-J. Prevent multiple instantiations of singleton objects 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 persists for the remainder of the Java Virtual Machine's (JVM) lifetime. Because the object is managed by a daemon thread, the thread cannot interfere with normal shutdown of the JVM.
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Starting and using background threads during class initialization can result in deadlock.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
TSM02-J |
Low |
Probable |
High | P2 | L3 |
Automated Detection
| Tool | Version | Checker | Description | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Parasoft Jtest |
| CERT.TSM02.CSTART | Do not call the "start" method of threads from inside a constructor | ||||||
| SonarQube |
| S2693 | Threads should not be started in constructors |
Bibliography
Chapter 8, "Lazy Initialization" | |
Chapter 5, "Creational Patterns, Singleton" |
Issue Tracking
| Tasklist | ||||
|---|---|---|---|---|
| ||||
||Completed||Priority||Locked||CreatedDate||CompletedDate||Assignee||Name||
|T|M|F|1269649993019|1269700561582|rcs_mgr|"Starting and using background threads during class initialization can result in class initialization cycles and deadlock. *For instance,* the main thread responsible for performing class initialization *may* block waiting for the background thread, which in turn will wait for the main thread to finish class initialization." ... see suggested words in bold...I am also generally unsure about the use of "can" vs. "may" because deadlocks are a "possibility" so perhaps "may" should be used?|
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