CON02-J. Facilitate thread reuse by using Thread Pools

The Thread-Per-Message design is the simplest concurrency technique wherein a thread is created for each incoming request. The benefits of creating a new thread to handle each request should outweigh the corresponding thread creation overheads. This design is generally recommended over sequential executions for time consuming, I/O bound, session based or isolated tasks.

On the other hand, there can be several disadvantages of this design such as creation overhead in case of frequent or recurring requests, significant processing overhead, resource exhaustion pertaining to threads (leading to the OutOfMemoryError), thread scheduling and context switching overhead [[Lea 00]].

Thread Pools overcome these disadvantages as the maximum number of worker threads that can be initiated and executed simultaneously, can be controlled. Every worker accepts a Runnable from a request and stores it in a temporary Channel like a buffer or a queue until resources become available. Since threads are reused and can be efficiently added to the Channel, most of the thread creation overhead is eliminated.

Noncompliant Code Example

This noncompliant code example demonstrates the Thread-Per-Message design that fails to provide graceful degradation of service.

class Helper {
  public void handle(String request) {
    //... 		
  }	
}

class GetRequest {
  protected final Helper h = new Helper();
  String request;

  public synchronized String accept() {
    String data = "Read data from pipe";
    //read the request data, else block
    return data;
  }

  public void request() {
    while(true) {
      request = accept();
      new Thread(new Runnable() {
        public void run() {
          h.handle(request);
        }
      }).start();
    }
  }
}

Compliant Solution

This compliant solution uses a fixed thread pool that places an upper bound on the number of simultaneously executing threads. Tasks submitted to the pool are stored in a internal queue. The system will not get overwhelmed trying to respond to all incoming requests but will degrade gracefully by serving a fixed number of clients at a particular time. [[Tutorials 08]]

According to [[API 06]] the java.util.concurrent Interface Executor:

[The Interface Executor is] An object that executes submitted Runnable tasks. This interface provides a way of decoupling task submission from the mechanics of how each task will be run, including details of thread use, scheduling, etc. An Executor is normally used instead of explicitly creating threads.

import java.util.concurrent.Executors;

class GetRequest {
  protected final Helper h = new Helper();
  String request;

  public synchronized String accept() {
    String data = "Read data from pipe";
    //read the request data, else block
    return data;
  }

  public void request() {
    int NoOfThreads = 200;
    Executor exec = (Executor) Executors.newFixedThreadPool(NoOfThreads);
    while(true) {
      request = accept();
      exec.Execute(new Runnable() {
        public void run() {
          h.handle(request);
        }
      });
    }
  }
}

Risk Assessment

Using simplistic concurrency primitives may lead to severe performance degradation and exhaustion of system resources.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[API 06]] Interface Executor
[[Lea 00]] Section 4.1.3 Thread-Per-Message and 4.1.4 Worker Threads
[[Tutorials 08]] Thread Pools

CON01-J. Avoid using ThreadGroup APIs      08. Concurrency (CON)      08. Concurrency (CON)