Page History

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Threads

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and

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tasks

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that

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block

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on

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operations

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involving

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network

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or file I/O must provide callers with an explicit termination mechanism to prevent denial-of-service (DoS) vulnerabilities.

Noncompliant Code Example (Blocking I/O, Volatile Flag)

This noncompliant code example uses a volatile done flag to indicate it whether is safe to shut down the thread, as suggested in THI05-J. Do not use Thread.stop() to terminate threads. However, when the thread is blocked on network I/O as a consequence of invoking the readLine() method, it cannot respond to the newly set flag until the network I/O is complete. Consequently, thread termination may be indefinitely delayed.

// Thread-safe class  file input or output (IO) must 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 if it is blocked on network IO as a consequence of invoking 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
  @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() {
    done = true;
  }

  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(); // Shut Shutdowndown 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.  The {{readData()}} and {{main()}} methods are modified as follows:

{code:bgColor=#FFcccc}
public final class SocketReader implements Runnable { // Thread-safe class
  // ...
  
  public void 

Noncompliant Code Example (Blocking I/O, Interruptible)

This noncompliant code example is similar to the preceding example but uses thread interruption to shut down the thread. Network I/O on a java.net.Socket is unresponsive to thread interruption.

// Thread-safe class 
public final class SocketReader implements Runnable { 
  // 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                throws IOException, InterruptedException {
    SocketReader reader = new SocketReader("somehost", 25);
    Thread thread = new Thread(reader);
    thread.start();
    Thread.sleep(1000); 
    thread.interrupt(); // Interrupt the thread
  }
}

Compliant Solution (Close Socket Connection)

This compliant solution terminates the blocking network I/O by closing the socket in the shutdown() method. The readLine() method throws a SocketException when the socket is closed, consequently allowing the thread to proceed. Note that it is impossible to keep the connection alive while simultaneously halting the thread both cleanly and immediately.

{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 {
    SocketReader reader = new SocketReader("somehost", 25             throws IOException, InterruptedException {
    SocketReader reader = new SocketReader("somehost", 25);
    Thread thread = new Thread(reader);
    thread.start();
    Thread.sleep(1000); 
    reader.shutdown();
  }
}
{code}

After {{

After the shutdown()

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method is

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called

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from

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main()

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,

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the

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finally

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block

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in

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readData()

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executes

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and

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calls

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shutdown()

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again,

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closing

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the

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socket

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for

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a

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second

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time.

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However,

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when the socket has already been closed, this second call does nothing.

When performing asynchronous I/O, a java.nio.channels.Selector

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can be unblocked by invoking either its close() or its wakeup() method.

When additional operations must be performed after emerging from the blocked state, use a boolean flag to indicate pending termination. When supplementing the code with such a flag, the shutdown() method should also set the flag to false so that the thread can cleanly exit from the while loop.

Compliant Solution (Interruptible Channel)

This compliant solution uses an interruptible channel, java.nio.channels.SocketChannel

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,

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instead

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of

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a

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Socket

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connection.

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If

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the

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thread

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performing

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the

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network I/O is interrupted using the Thread.interrupt()

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method

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while

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it

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is

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reading

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the

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data,

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the

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thread

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receives

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a

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ClosedByInterruptException, and the channel is closed immediately. The thread's

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interrupted

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status

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is

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also

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set.

}
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 IOException, InterruptedException {
    SocketReader reader = new SocketReader("somehost", 25);
    Thread thread = new Thread(reader);
    thread.start();
    Thread.sleep(1000);
    thread.interrupt();
  }
}
{code}

This

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technique

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interrupts

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the

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current

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thread. However,

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it

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stops

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the

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thread

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only because

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the

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code

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polls

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the

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thread's

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interrupted

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status

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with

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the

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Thread.interrupted()

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method

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and terminates the thread when it is interrupted. Using a SocketChannel ensures that the condition in the while loop is tested as soon as an interruption is received, even though the read is normally a blocking operation. Similarly, invoking the interrupt() method of a thread blocked on a java.nio.channels.Selector

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also

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causes

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that

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thread

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to

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awaken.

Noncompliant Code Example (Database Connection)

This noncompliant code example shows a thread-safe DBConnector class 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 intended to be single-threaded.

public final class DBConnector implements  


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 =  "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}

Database

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connections,

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like

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sockets,

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lack inherent interruptibility. Consequently,

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this

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design fails to support the client's attempts to cancel a task by closing the resource when the corresponding thread is blocked on a long-running query, such as a join.

Compliant Solution (Statement.cancel()

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)

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This

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compliant

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solution

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uses

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a

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ThreadLocal

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wrapper

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around

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the

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connection

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so

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that

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a thread calling the initialValue() method obtains a unique connection instance. This approach allows provision of a cancelStatement() so that other threads or clients can interrupt a long-running query when required. The cancelStatement() method invokes the Statement.cancel() method.

Code Block
bgColor #ccccff
thread that calls {{initialValue()}} obtains a unique connection instance. The advantage of this approach is that a {{cancelStatement()}} method can be provided so that other threads or clients can interrupt a long running query when required. The {{cancelStatement()}} method invokes {{Statement.cancel()}}. {code:bgColor=#ccccff} public final class DBConnector implements Runnable { private final String query; private volatile Statement stmt; DBConnector(String query) { this.query = query; try {} private static final ThreadLocal<Connection> stmtconnectionHolder = getConnection().createStatement(); } catch (SQLException e) { // Forward to handler } } private static 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) { // Forward to handler } return connection; } }; public Connection getConnection() { return connectionHolder.get(); } public boolean cancelStatement() { // Allows client to cancel statement Statement tmpStmt = stmt; if (stmttmpStmt != null) { try { stmt tmpStmt.cancel(); return true; } catch (SQLException e) { // Forward to handler } } return false; } @Override public void run() { try { if (stmt.getConnection() != getConnection(null)) { throwstmt new IllegalStateException= getConnection().createStatement(); }   if ResultSet rs = stmt.executeQuery(query); // ... } (stmt == null || (stmt.getConnection() != getConnection())) { throw new IllegalStateException(); } 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.cancelStatement(); } } {code} The {{Statement

The Statement.cancel()

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method

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cancels

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the

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query

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, provided

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the database management system (DBMS) and driver both support cancellation. It is impossible to cancel the query if either the DBMS or the driver fail to support cancellation.

According to the Java API, Interface Statement documentation [API 2014]

By default, only one ResultSet object per Statement object can be open at the same time. As a result, if the reading of one ResultSet object is interleaved with the reading of another, each must have been generated by different Statement objects.

This compliant solution ensures that only one ResultSet is associated with the Statement belonging to an instance, and, consequently, only one thread can access the query results.

Risk Assessment

Failure to provide facilities for thread termination can cause nonresponsiveness and DoS.

Bibliography

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