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Many programs must address the problem of handling a series of incoming requests. The Thread-Per-Message design pattern is the simplest concurrency strategy wherein a new thread is created for each request \[[Lea 2000|AA. Bibliography#Lea 00]\].  This pattern is generally preferred to sequential executions of time-consuming, I/O-bound, session-based, or isolated tasks.

However, this pattern also has several pitfalls, including overheads of thread-creation and scheduling, task processing, resource allocation and deallocation, and frequent context switching \[[Lea 2000|AA. Bibliography#Lea 00]\]. Furthermore, an attacker can cause a denial of service by overwhelming the system with too many requests all at once. Instead of degrading gracefully, the system becomes unresponsive, causing a denial of service.  From a safety perspective, one component can exhaust all resources because of some intermittent error, starving all other components.

class Helper {
  public void handle(Socket socket) {
    //...
  }
}

final class RequestHandler {
  private final Helper helper = new Helper();
  private final ServerSocket server;

  private RequestHandler(int port) throws IOException {
    server = new ServerSocket(port);
  }

  public static RequestHandler newInstance() throws IOException {
    return new RequestHandler(0); // Selects next available port
  }

  public void handleRequest() {
    new Thread(new Runnable() {
      public void run() {
        try {
          helper.handle(server.accept());
        } catch (IOException e) {
          // Forward to handler
        }
      }
    }).start();
  }

}

This compliant solution uses a fixed-thread pool that places an upper bound on the number of concurrently executing threads. Tasks submitted to the pool are stored in an internal queue. This prevents the system from being overwhelmed when trying to respond to all incoming requests and allows it to degrade gracefully by serving a fixed number of clients at a particular time \[[Tutorials 2008|AA. Bibliography#Tutorials 08]\].

// class Helper remains unchanged

final class RequestHandler {
  private final Helper helper = new Helper();
  private final ServerSocket server;
  private final ExecutorService exec;

  private RequestHandler(int port, int poolSize) throws IOException {
    server = new ServerSocket(port);
    exec = Executors.newFixedThreadPool(poolSize);
  }

  public static RequestHandler newInstance(int poolSize) throws IOException {
    return new RequestHandler(0, poolSize);
  }

  public void handleRequest() {
    Future<?> future = exec.submit(new Runnable() {
      @Override public void run() {
	try {
  	  helper.handle(server.accept());
	} catch (IOException e) {
          // Forward to handler
        }
      }
    });
  }
  // ... other methods such as shutting down the thread pool and task cancellation ...
}

According to the Java API documentation for the {{Executor}} interface \[[API 2006|AA. Bibliography#API 06]\]:

The choice of the unbounded {{newFixedThreadPool}} is not always optimal. Refer to the Java API documentation for choosing between the following  to meet specific design requirements \[[API 2006|AA. Bibliography#API 06]\]: 

\[[API 2006|AA. Bibliography#API 06]\] [Interface Executor|http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/Executor.html]
\[[Lea 2000|AA. Bibliography#Lea 00]\] Section 4.1.3 Thread-Per-Message and 4.1.4 Worker Threads
\[[Tutorials 2008|AA. Bibliography#Tutorials 08]\] [Thread Pools|http://java.sun.com/docs/books/tutorial/essential/concurrency/pools.html]
\[[Goetz 2006|AA. Bibliography#Goetz 06]\] Chapter 8, Applying Thread Pools
\[[MITRE 2009|AA. Bibliography#MITRE 09]\] [CWE ID 405|http://cwe.mitre.org/data/definitions/405.html] "Asymmetric Resource Consumption (Amplification)", [CWE ID 410|http://cwe.mitre.org/data/definitions/410.html] "Insufficient Resource Pool"