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Wiki MarkupStarting and using background threads during class initialization can result in class initialization cycles and eventually, deadlock. This is because 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. This issue can arise, for example, when a database connection is established in a background thread while class initialization is underway. \[[Bloch 05b|AA. Java References#Bloch 05b]\]

Noncompliant Code Example

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.

Noncompliant Code Example (Background Thread)

In this noncompliant code example, the static initializer starts a background thread as part of class initializationThis 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 should wait until initialization of all members of the Lazy ConnectionFactory class, including dbConnection, has finishedare initialized.

public final class LazyConnectionFactory {
  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);
    }
    // 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 = getConnection();
  }
}

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. \[[Bloch 05b|AA. Java References#Bloch 05b]\] 

Statically initialized fields are guaranteed to be fully constructed before they are made visible to other threads (see 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 TSM01Similar 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). Creating timers that perform recurring tasks and starting those timers from within code responsible for initialization also introduces liveness issues.

Compliant Solution (

...

Static Initializer, No Background Threads)

This compliant solution also uses a static initializer for initialization and does not spawn a background thread from itinitializes all fields on the main thread rather than spawning background threads from the static initializer.

public final class LazyConnectionFactory {
  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   // Other initializationany threads)
  }

  // ...
}

Compliant Solution (ThreadLocal)

This compliant solution uses initializes the database connection from a ThreadLocal object to initialize the database connection so that every each thread can obtain its own unique instance of the connection.

public final class LazyConnectionFactory {
  private static final ThreadLocal<Connection> connectionHolder
                       = new ThreadLocal<Connection>() {
     @Override public Connection initialValue() {
     try {
      try {
Connection dbConnection =
        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();
  }
}

It is also safe to set other shared class variables The static initializer can be used to initialize any shared class field. Alternatively, the fields can be initialized from the initialValue() method.

Exceptions

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);
  }
}

This is a singleton class (see CON23MSC07-J. Address the shortcomings of the Singleton design patternPrevent multiple instantiations of singleton objects for more information on how to defensively code singleton classes). The initialization creates involves creating a background thread referencing the object, and the thread itself waits indefinitelyusing the current instance of the class. The thread waits indefinitely by invoking Object.wait(). Consequently, this object exists persists for the remainder of the JVMJava Virtual Machine's lifetime; however, as it (JVM) lifetime. Because the object is managed by a daemon thread, the thread (and object) do not hinder a cannot interfere with normal shutdown of the JVM.

While Although the initialization does involve involves a background thread, the background thread does not access any fields, and creates no that thread neither accesses fields nor creates any liveness or safety issues. Consequently, this code is a safe and useful exception to this guidelinerule.

Risk Assessment

Starting and using background threads during class initialization can result in deadlock conditions.

Automated Detection

...

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[Bloch 05b|AA. Java References#Bloch 05b]\] 8. "Lazy Initialization"
\[[Patterns 02|AA. Java References#Patterns 02]\] Chapter 5, Creational Patterns, Singleton

Tool	Version	Checker	Description
Parasoft Jtest	Include Page Parasoft_V Parasoft_V	CERT.TSM02.CSTART	Do not call the "start" method of threads from inside a constructor
SonarQube	Include Page SonarQube_V SonarQube_V	S2693	Threads should not be started in constructors

Bibliography

Issue Tracking

||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?|

...

Image Added Image Added Image AddedCON02-J. Always synchronize on the appropriate object      11. Concurrency (CON)      CON04-J. Synchronize using an internal private final lock object