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

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 initialization. The background thread attempts to initialize a database connection but should wait until all members of the ConnectionFactory class, including dbConnection, are initializedThis noncompliant code example begins initializing the class Lazy.

public final class LazyConnectionFactory {
  private static intConnection number;
dbConnection;
  // Other fields ...

  static {
    Thread tdbInitializerThread = new Thread(new Runnable() {
        @Override public void run() {
          // Initialize, for example, a database connection
 the database connection
          try {
            dbConnection = DriverManager.getConnection("connection string");
          } catch (SQLException e) {
         Lazy.this.number   dbConnection = 42;
null;
          }
        }
    });

    // Other initialization, for example, start other threads

    tdbInitializerThread.start();
    try {
      tdbInitializerThread.join();
    } catch (InterruptedException ie) {
        throw new AssertionError(ie);
    }
  }

  //public Otherstatic initialization
Connection getConnection() }{
  public static voidif main(String[] args(dbConnection == null) {
     System.out.println(number);
  }
}

The code in the {{static}} block is responsible for initialization, and starts a background thread. The background thread attempts to assign to the {{number}} 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. \[[Bloch 05b|AA. Java References#Bloch 05b]\] 

 throw new IllegalStateException("Error initializing connection");
    }
    return dbConnection;
  }

  public static void main(String[] args) {
    // ...
    Connection connection = getConnection();
  }
}

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 but 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 intConnection numberdbConnection;
  // Other fields ...

  static {
    // Initialize, for example, a database connection
     this.number = 42try {
      dbConnection = DriverManager.getConnection("connection string");
    } catch (SQLException e) {
   
   dbConnection public= staticnull;
 void main(String[] args) {}
     System.out.println(number);// Other initialization (do not start any threads)
  }

  // ...
}

Compliant Solution (ThreadLocal)

This compliant solution uses initializes the database connection from a ThreadLocal object to initialize a database connection and sets flag to true depending on whether the initialization succeedsso that each thread can obtain its own unique instance of the connection.

public final class LazyConnectionFactory {
  private static final booleanThreadLocal<Connection> flag;

connectionHolder
               private static final ThreadLocal<Connection> connectionHolder
    = new ThreadLocal<Connection>() {
   @Override   public Connection initialValue() {
        try {
          Connection conndbConnection = DriverManager.getConnection("connectionstring");
          flag = true DriverManager.getConnection("connection string");
          return conndbConnection;
        } catch (SQLException e) {
       return null;
     }
  flag =}
 false};

  // Other fields ...

  static {
   return null;
       // Other initialization (do not start any threads)
  }

  public static Connection getConnection() }{
    }Connection connection = connectionHolder.get();
    if (connection == null) {
     public staticthrow Connectionnew getConnection() {IllegalStateException("Error initializing connection");
    }
    return connectionHolder.get()connection;
  }

  public static void main(String[] args) {
    Connection conn = getConnection();
    System.out.println(flag// ...
    Connection connection = getConnection();
  }
}

It is safe to set 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. Consequently, each thread will see a consistent value of the flag field.

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(); // keepKeep this object alive
  }
 
  // Neither this class, nor HashSetHashMap 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) {
    /* Forward to handler */  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 pattern for how to properly handle Prevent 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 forever. Consequently this object exists using the current instance of the class. The thread waits indefinitely by invoking Object.wait(). Consequently, this object 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) will not hinder a cannot interfere with normal shutdown of the JVM.

While Although the initialization does involve involves a background thread, the background that thread neither accesses no fields and so creates no deadlocknor creates any liveness or safety issues. Consequently, this code is a safe and useful exception to this rule.

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 lock object