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// Thread-safe class 
public final class SocketReader implements Runnable { 
  private final Socket socket;
  private final BufferedReader in;
  private volatile boolean done = false;
  private final Object lock = new Object();

  public SocketReader(String host, int port) throws IOException {
    this.socket = new Socket(host, port);
    this.in = new BufferedReader(
        new InputStreamReader(this.socket.getInputStream())
    );
  }

  // Only one thread can use the socket at a particular time
  @Override public void run() {
    try {
      synchronized (lock) {
        readData();
      }
    } catch (IOException ie) {
      // Forward to handler
    }
  }

  public void readData() throws IOException {
    String string;
    while (!done && (string = in.readLine()) != null) {
      // Blocks until end of stream (null)
    }
  }

  public void shutdown(

The recommendations suggested in the guideline [CON13-J. Do not use Thread.stop() to terminate threads] are insufficient to terminate a thread that is blocked on an operation involving network or file input or output (IO). Threads and tasks should provide callers with an explicit termination mechanism to prevent denial of service vulnerabilities.


h2. Noncompliant Code Example (blocking IO, volatile flag)

This noncompliant code example uses a volatile {{done}} flag to indicate that it is safe to shut down the thread, as suggested in [CON13-J. Do not use Thread.stop() to terminate threads]. However, setting the flag does not terminate the thread because the thread is blocked on network IO as a consequence of using the {{readLine()}} method. 

{code:bgColor=#FFcccc}
public final class SocketReader implements Runnable { // Thread-safe class
  private final Socket socket;
  private final BufferedReader in;
  private volatile boolean done = false;
  private final Object lock = new Object();

  public SocketReader(String host, int port) throws IOException {
    this.socket = new Socket(host, port);
    this.in = new BufferedReader(new InputStreamReader(this.socket.getInputStream()));
  }
  
  // Only one thread can use the socket at a particular time
  public void run() {
    try {
      synchronized (lock) {
        readData(); 
      }      
    } catch (IOException ie) {
    done  // Forward to handler
    }= true;
  }

  public static void readDatamain() throws IOException {String[] args) 
    String string;
    while (!done && (string = in.readLine()) != null) {
      // Blocks until endthrows ofIOException, streamInterruptedException (null){
    }
SocketReader reader }

=  public void shutdown() {
new SocketReader("somehost", 25);
    Thread donethread = true new Thread(reader);
    }
thread.start();
  public static void main(String[] args) throws IOException, InterruptedException {
    SocketReader reader = new SocketReader("somehost", 25);
    Thread thread = new Thread(reader);
    thread.start();
    Thread.sleep(1000);
    reader.shutdown(); // Shutdown the thread
  }
}
{code}


h2. Noncompliant Code Example (blocking IO, interruptible)

This noncompliant code example is similar to the preceding one, but uses thread interruption to shut down the thread. Network IO is not responsive to thread interruption when a {{java.net.Socket}} is being used. 

{code:bgColor=#FFcccc}
public final class  Thread.sleep(1000);
    reader.shutdown(); // Shut down the thread
  }
}

// Thread-safe class 
public final class SocketReader implements Runnable { // Thread-safe class
  // Other methods...
  
  public void readData() throws IOException {
    String string;
    while (!Thread.interrupted() && (string = in.readLine()) != null) { 
      // Blocks until end of stream (null)
    }
  }
  
  public static void main(String[] args) 
                          throws IOException, InterruptedException {
    SocketReader reader = new SocketReader("somehost", 25);
    Thread thread = new Thread(reader);
    thread.start();
    Thread.sleep(1000); 
    thread.interrupt(); // Interrupt the thread
  }
}

{code}


h2. Compliant Solution (close socket connection)

This compliant solution resumes the thread by having the {{shutdown()}} method close the socket. The {{readLine()}} method throws a {{SocketException}} when the socket is closed which lets the thread proceed. Note that there is no way to keep the connection alive if the thread is to be cleanly halted immediately.

{code:bgColor=#ccccff}
public final class SocketReader implements Runnable {
  // Other methods...
  
  public void readData() throws IOException {
    String string;
    try {
      while ((string = in.readLine()) != null) { 
        // Blocks until end of stream (null)
      }
    } finally {
      shutdown();
    }
  }
  
  public void shutdown() throws IOException {
    socket.close();
  }

  public static void main(String[] args) 
 throws IOException, InterruptedException                        throws IOException, InterruptedException {
    SocketReader reader = new SocketReader("somehost", 25);
    Thread thread = new Thread(reader);
    thread.start();
    Thread.sleep(1000); 
    reader.shutdown();
  }
}
{code}

The {{finally}} block executes after {{

 {{ClosedByInterruptException}} and the channel is closed immediately. The thread's interrupt status is also set.

{code:bgColor=#ccccff}
public final class SocketReader implements Runnable {
  private final SocketChannel sc;
  private final Object lock = new Object();
  
  public SocketReader(String host, int port) throws IOException {
    sc = SocketChannel.open(new InetSocketAddress(host, port));    
  }

  
 @Override public void run() {
    ByteBuffer buf = ByteBuffer.allocate(1024);
    try {
      synchronized (lock) {
        while (!Thread.interrupted()) {
          sc.read(buf);
          // ...
        }
      }
    } catch (IOException ie) {
      // Forward to handler
    }
  }

  public static void main(String[] args) 
 throws                          throws IOException, InterruptedException {
    SocketReader reader = new SocketReader("somehost", 25);
    Thread thread = new Thread(reader);
    thread.start();
    Thread.sleep(1000);
    thread.interrupt();
  }
}
{code}

 


h2. Noncompliant Code Example (database connection)

This noncompliant code example shows a thread-safe class {{DBConnector}} that creates one JDBC connection per thread.  Each connection belongs to one thread, and is not shared by other threads. This is a common use-case because JDBC connections are not meant to be shared by multiple-threads. 

{code:bgColor=#FFcccc}
public final class DBConnector implements Runnable {
  private final String query;
  
  DBConnector(String query) {
    this.query = query; 
  }
	
  @Override public void run() {
    Connection connection;
    try {
      // Username and password are hard coded for brevity
      connection = DriverManager.getConnection("
          "jdbc:driver:name", 
          "username", 
          "password"
      );  
      Statement stmt = connection.createStatement();
      ResultSet rs = stmt.executeQuery(query);
      // ...
    } catch (SQLException e) {
      // Forward to handler
    }
    // ... 
  }  

  public static void main(String[] args) throws InterruptedException {
    DBConnector connector = new DBConnector("suitable query");
    Thread thread = new Thread(connector);
    thread.start();
    Thread.sleep(5000);
    thread.interrupt();
  }
}
{code}

Unfortunately database 

public final class DBConnector implements Runnable {
  private final String query;
  private volatile Statement stmt;

  DBConnector(String query) {
    this.query = query;
  }

  private static final ThreadLocal<Connection> connectionHolder = 
                                       new ThreadLocal<Connection>() {
    Connection connection = null;

    @Override public Connection initialValue() {
      try {
        // ...
        connection = DriverManager.getConnection(
            "jdbc:driver:name", 
            "username", 
            "password"
        );
      } catch (SQLException e) design does not permit a client to cancel a task by closing the resource if the corresponding thread is blocked on a long running activity such as a join query. 

{mc}Not sure what to do with the following text. ~DS :
Other than helping thread cancellation, a mechanism to close connections also prevents thread starvation because threads can be allowed to fairly share the limited number of database connections available in the pool. A similar mechanism is required for task cancellation when using socket objects that are local to a method. {mc}


h2. Compliant Solution (ThreadLocal)

This compliant solution uses a {{ThreadLocal}} wrapper around the connection so that a thread that calls {{initialValue()}} obtains a unique connection instance. The advantage of this approach is that a {{shutdownConnection()}} method can be provided so that clients external to the class can also close the connection when the corresponding thread is blocked, or is performing some time consuming activity.

{code:bgColor=#ccccff}
public final class DBConnector implements Runnable {
  private final String query;
  
  DBConnector(String query) {
    this.query = query;
  }
	
  private static ThreadLocal<Connection> connectionHolder = new ThreadLocal<Connection>() {
    Connection connection = null;

    @Override public Connection initialValue() {		
      try {
        // Username and password are hard coded for brevityForward to handler
      }
      return  connection = DriverManager.getConnectionconnection;
    }
  };

  public Connection ("jdbc:driver:name", "username", "password");  	    	      
      } catch (SQLException e) {
        // Forward to handler
getConnection() {
    return connectionHolder.get();
  }

  public boolean cancelStatement() { // Allows client to cancel statement
    Statement tmpStmt = stmt;
    if (tmpStmt != null) {
      try }{
       return connectiontmpStmt.cancel();
        return }
		
true;
     @Override public} voidcatch set(ConnectionSQLException cone) {
      if(connection == null) { // ShutsForward downto connectionhandler
 when con = null		   }
    }
    return false;
  }

  @Override public void run() {
    try {
      if (getConnection() !=  connection.close();null) {
        }stmt catch= (SQLException e) {getConnection().createStatement();
      }
     // Forwardif to(stmt handler== 
null || (stmt.getConnection() != getConnection())) {
   }	     throw  		       new IllegalStateException();
      } else {
      ResultSet rs connection = con; 
stmt.executeQuery(query);
      // }...
    } catch (SQLException e) {
       };

  public static Connection getConnection() {// Forward to handler
    }
    return// connectionHolder.get();...
  }

  public static void shutdownConnectionmain(String[] args) throws InterruptedException {
 // Allows client toDBConnector closeconnector connection= anytime
    connectionHolder.set(nullnew DBConnector("suitable query");
   }

 Thread thread public= voidnew runThread(connector) {;
    Connection dbConnection = getConnectionthread.start();
    Statement stmtThread.sleep(5000);
    try {
      stmt = dbConnection.createStatement();		
      ResultSet rs = stmt.executeQuery(query);
      // ...
    } catch (SQLException e) {
      // Forward to handler	 
    }    
    // ...
  }

  public static void main(String[] args) throws InterruptedException {
    DBConnector connector = new DBConnector("suitable query");
    Thread thread = new Thread(connector);
    thread.start();
    Thread.sleep(5000);
    connector.shutdown();
  }
}
{code}


h2. Risk Assessment

Failing to provide facilities for thread shutdown can cause non-responsiveness and denial of service.

|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| CON24- J | low | probable | medium | {color:green}{*}P4{*}{color} | {color:green}{*}L3{*}{color} |



h3. Automated Detection

TODO



h3. Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the [CERT website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+CON24-J].

h2. References

\[[API 06|AA. Java References#API 06]\] Class Thread, method {{stop}}, interface ExecutorService
\[[Darwin 04|AA. Java References#Darwin 04]\] 24.3 Stopping a Thread
\[[JDK7 08|AA. Java References#JDK7 08]\] Concurrency Utilities, More information: Java Thread Primitive Deprecation 
\[[JPL 06|AA. Java References#JPL 06]\] 14.12.1. Don't stop and 23.3.3. Shutdown Strategies
\[[JavaThreads 04|AA. Java References#JavaThreads 04]\] 2.4 Two Approaches to Stopping a Thread
\[[Goetz 06|AA. Java References#Goetz 06]\] Chapter 7: Cancellation and shutdown

----
[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_left.png!|CON23-J. Address the shortcomings of the Singleton design pattern]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_up.png!|11. Concurrency (CON)]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_right.png!|CON25-J. Ensure atomicity when reading and writing 64-bit values]

connector.cancelStatement();
  }
}