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

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file

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I/O must provide callers with an explicit termination mechanism to prevent denial-of-service

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(DoS) vulnerabilities.

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Noncompliant

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Code

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Example

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(Blocking

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I/O,

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Volatile

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Flag)

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This

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noncompliant

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code

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example

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uses

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a

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volatile

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done

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flag

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to

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indicate

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it

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whether is

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safe

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to

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shut

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down

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the

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thread,

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as

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suggested

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in

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THI05-J.

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Do

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not

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use

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Thread.stop()

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to

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terminate

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threads

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.

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However,

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when the thread is blocked on network I/O

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as

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a

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consequence

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of

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invoking

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the

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readLine() method, it cannot respond to the newly set flag until the network I/O is complete. Consequently, thread termination may be indefinitely delayed.

Code Block
bgColor #FFcccc
// Thread-safe class }} 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

Noncompliant Code Example (Blocking

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I/O,

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Interruptible)

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This

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noncompliant

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code

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example

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is

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similar

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to

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the

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preceding

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example but

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uses

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thread

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interruption

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to

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shut

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down

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the

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thread.

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Network

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I/O on a java.net.Socket is unresponsive to thread interruption.

Code Block
bgColor #FFcccc
// Thread-safe class public final class SocketReader implements Runnable { // Other methods 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 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);
    Thread thread = new Thread(reader);
    thread.start();
    Thread.sleep(1000);
    reader.shutdown();
  }
}
{code}

After the {{

After the shutdown()

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method

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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,

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a

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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

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I/O

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is

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interrupted

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using

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the

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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

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,

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and

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the

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channel

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is

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closed

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immediately.

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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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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 technique interrupts the current thread. However, it only stops the thread because the code polls the thread's interrupted status with the {{

This technique interrupts the current thread. However, it stops the thread only because the code polls the thread's interrupted status with the Thread.interrupted()

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method

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and

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terminates

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the

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thread

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when

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it

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is

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interrupted.

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Using

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a

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SocketChannel

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ensures

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that

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the

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condition

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in

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the

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while

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loop

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is

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tested

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as

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soon

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as

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an

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interruption

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is

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received,

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even though the

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read

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is normally a

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blocking

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operation.

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Similarly,

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invoking

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the

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interrupt()

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method

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of

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a

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thread

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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 


h2. Noncompliant Code Example (database connection)

This noncompliant code example shows a thread-safe {{DBConnector}} class that creates one Java Database Connectivity (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}

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 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())

This compliant solution uses a ThreadLocal wrapper around the connection so that 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.

 design does not permit a client to cancel a task by closing the resource if the corresponding thread is blocked on a long-running query such as a join.


h2. Compliant Solution ({{Statement.cancel()}})

This compliant solution uses a {{ThreadLocal}} wrapper around the connection so that a thread that calling the {{initialValue()}} method 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 the {{Statement.cancel()}} method.

{code:bgColor=#ccccff}
public final class DBConnector implements Runnable {
  private final String query;
  private volatile Statement stmt;

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

  if(getConnection() != null) {
      try {
        stmt = getConnection().createStatement();private static final ThreadLocal<Connection> connectionHolder = 
      } catch (SQLException e) {
        // Forward to handler
      }
    }
  }

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

    @Override public Connection initialValue() {
      try {
        // ...
        connection = DriverManager.getConnection("jdbc:driver:name", "username", 
            "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 {
        stmttmpStmt.cancel();
        return true;
      } catch (SQLException e) {
        // Forward to handler
      }
    }
    return false;
  }

  @Override public void run() {
    try {
      if (stmtgetConnection() != null) {
        stmt = getConnection().createStatement();
      }
      if (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 {{

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

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database

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management

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system

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(DBMS)

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and

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driver

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both

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support

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cancellation.

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It

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is

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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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