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public final class ValidationService {
  // ...
  public StringBuilder fieldAggregator(String... inputs)
      throws InterruptedException, ExecutionException {
    // ...
    for (int i = 0; i < inputs.length; i++) {
      // Don't pass-in thread pool  
      results[i] = pool.submit(new ValidateInput<String>(inputs[i])); 
    }
    // ...
  }
}

// Does not use same thread pool
public final class ValidateInput<V> implements Callable<V> { 
  private final V input;

  ValidateInput(V input) {
    this.input = input;
  }

  @Override public V call() throws Exception {
    // ...
If validation fails, throw foran (intexception ihere
 = 0; i < inputs.length; i++) {
     return (V) new SanitizeInput().sanitize(input);
  }
}

public final class SanitizeInput<V> {  // Don't pass-in thread pool  No longer a Callable task
  public SanitizeInput() {}

  results[i] = pool.submit(new ValidateInput<String>(inputs[i])); public V sanitize(V input) {
    }
    // ...
Sanitize  }
}

// Does not use same thread pool
public final class ValidateInput<V> implements Callable<V> { 
  private final V input;

  ValidateInput(V input) {
    this.input = input;
  }

  @Override public V call() throws Exception {
    // If validation fails, throw an exception here
    return (V) new SanitizeInput().sanitize(input);
  }
}

public final class SanitizeInput<V> {  // No longer a Callable task
  public SanitizeInput() {}

  public V sanitize(V input) {
    // Sanitize input and return
    return input;
  }
}
{code}

Thread-starvation issues can be partially mitigated by choosing a large thread pool size. However, an untrusted caller can still overwhelm the system by supplying more inputs. (See rule [TPS00-J. Use thread pools to enable graceful degradation of service during traffic bursts].)

Note that operations that have further constraints, such as the total number of database connections or total {{ResultSet}} objects open at a particular time, impose an upper bound on the usable thread pool size as each thread continues to block until the resource becomes available.

Private static {{ThreadLocal}} variables may be used to maintain local state in each thread. When using thread pools, the lifetime of {{ThreadLocal}} variables should be bounded by the corresponding task \[[Goetz 2006|AA. References#Goetz 06]\]. Furthermore, programs must not use these variables to communicate between tasks. There are additional constraints in the use of {{ThreadLocal}} variables in thread pools; see rule [TPS04-J. Ensure ThreadLocal variables are reinitialized when using thread pools] for more information.


h2. Noncompliant Code Example (Sub-tasks)

{mc} 
h2. Compliant Solution (Unbounded Thread Pool)
This compliant solution uses a cached thread pool, which dynamically creates new threads as needed and prevents deadlock. However, this implementation can result in resource exhaustion and should not be used in front-end or critical production systems. 

{code:bgColor=#ccccff} 
public final class ValidationService { 
private final ExecutorService pool; 

public ValidationService(int poolSize) { 
pool = Executors.newCachedThreadPool(); 
} 
// ... 
} 
{code} 

According to the Java API \[[API 2006|AA. Java References#API 06]\], the {{Executors.newCachedThreadPool()}} method 

{quote} 
Creates a thread pool that creates new threads as needed, but will reuse previously constructed threads when they are available. These pools will typically improve the performance of programs that execute many short-lived asynchronous tasks. Calls to execute will reuse previously constructed threads if available. If no existing thread is available, a new thread will be created and added to the pool. Threads that have not been used for sixty seconds are terminated and removed from the cache. Consequently, a pool that remains idle for long enough will not consume any resources. 
{quote} 
{mc}

This noncompliant code example contains a series of subtasks that execute in a shared thread pool  \[[Gafter 2006|AA. References#Gafter 06]\].  The {{BrowserManager}} class calls {{perUser()}}, which starts tasks that invoke {{perProfile()}}. The {{perProfile()}} method starts tasks that invoke {{perTab()}}, and in turn, {{perTab}} starts tasks that invoke {{doSomething()}}.  {{BrowserManager}} then waits for the tasks to finish. The threads are allowed to invoke {{doSomething()}} in any order, provided that {{count}} correctly records the number of methods executed.


{code:bgColor=#FFCCCC}
public final class BrowserManager {
  private final ExecutorService pool = Executors.newFixedThreadPool(10);
  private final int numberOfTimes;
  private static AtomicInteger count = new AtomicInteger(); // count = 0

  public BrowserManager(int n) {
    numberOfTimes = n;
  }

  public void perUser() {
    methodInvoker(numberOfTimes, "perProfile");
    pool.shutdown();
  }

  public void perProfile() {
    methodInvoker(numberOfTimes, "perTab");
  }

  public void perTab() {
    methodInvoker(numberOfTimes, "doSomething");
  }

  public void doSomething() {
    System.out.println(count.getAndIncrement());
  }

  public void methodInvoker(int n, final String method) {
    final BrowserManager manager = this;
    Callable<Object> callable = new Callable<Object>() {
      @Override public Object call() throws Exception {
        Method meth = manager.getClass().getMethod(method);
        return meth.invoke(manager);
      }
    };

    Collection<Callable<Object>> collection = 
        Collections.nCopies(n, callable);
    try {
      Collection<Future<Object>> futures = pool.invokeAll(collection);
    } catch (InterruptedException e) {
      // Forward to handler
      Thread.currentThread().interrupt(); // Reset interrupted status
    }
    // ...
  }

  public static void main(String[] args) {
    BrowserManager manager = new BrowserManager(5);
    manager.perUser();
  }
}
{code}

Unfortunately, this program is susceptible to a thread-starvation deadlock. For example, if each of the five {{perUser}} tasks spawns five {{perProfile}} tasks, where each {{perProfile}} task spawns a {{perTab}} task, the thread pool will be exhausted, and {{perTab()}} will be unable to allocate any additional threads to invoke the {{doSomething()}} method.


h2. Compliant Solution ({{CallerRunsPolicy}})

{mc}To prevent thread starvation, every level (worker) must have a double-ended queue, where all sub-tasks are queued \[[Goetz 2006|AA. Java References#Goetz 06]\]. Each level removes the most recently generated sub-task from the queue so that it can process it. When there are no more threads left to process, the current level runs the least-recently created sub-task of another level by picking and removing it from that level's queue (work stealing). {mc}

This compliant solution selects and schedules tasks for execution, avoiding thread-starvation deadlock. It sets the {{CallerRunsPolicy}} on a {{ThreadPoolExecutor}} and uses a {{SynchronousQueue}} \[[Gafter 2006|AA. References#Gafter 06]\]. The policy dictates that when the thread pool runs out of available threads, any subsequent tasks will run in the thread that submitted the tasks.

{code:bgColor=#ccccff}
public final class BrowserManager {
  private final static ThreadPoolExecutor pool =
      new ThreadPoolExecutor(0, 10, 60L, TimeUnit.SECONDS,
                             new SynchronousQueue<Runnable>());
  private final int numberOfTimes;
  private static AtomicInteger count = new AtomicInteger(); // count = 0

  static {
    pool.setRejectedExecutionHandler(
    new ThreadPoolExecutor.CallerRunsPolicy());
  }

  // ...
}
{code}

According to Goetz and colleagues \[[Goetz 2006|AA. References#Goetz 06]\]

{quote}
A {{SynchronousQueue}} is not really a queue at all, but a mechanism for managing handoffs between threads. In order to put an element on the {{SynchronousQueue}}, another thread must already be waiting to accept the handoff. If no thread is waiting, but the current pool size is less than the maximum, {{ThreadPoolExecutor}} creates a new thread; otherwise, the task is rejected according to the saturation policy.
{quote}

According to the Java API \[[API 2006|AA. References#API 06]\], the {{CallerRunsPolicy}} class is

{quote}
A handler for rejected tasks that runs the rejected task directly in the calling thread of the {{execute}} method, unless the executor has been shut down, in which case, the task is discarded.
{quote}

In this compliant solution, tasks that have other tasks waiting to accept the hand-off are added to the {{SynchronousQueue}} when the thread pool is full. For example, tasks corresponding to {{perTab()}} are added to the {{SynchronousQueue}} because the tasks corresponding to {{perProfile()}} are waiting to receive the hand-off. Once the pool is full, additional tasks are rejected according to the saturation policy in effect. Because the {{CallerRunsPolicy}} is used to handle these rejected tasks, all the rejected tasks are executed in the main thread that started the initial tasks. When all the threads corresponding to {{perTab()}} have finished executing, the next set of tasks corresponding to {{perProfile()}} are added to the {{SynchronousQueue}} because the handoff is subsequently used by {{perUser()}} tasks. 

The {{CallerRunsPolicy}} allows graceful degradation of service when faced with many requests by distributing the workload from the thread pool to the work queue. Because the submitted tasks cannot block for any reason other than waiting for other tasks to complete, the policy guarantees that the current thread can handle multiple tasks sequentially. The policy would fail to prevent thread-starvation deadlock if the tasks were to block for some other reason, such as network I/O. Furthermore, this approach avoids unbounded queue growth because {{SynchronousQueue}} avoids storing tasks indefinitely for future execution, and all tasks are handled either by the current thread or by a thread in the thread pool.

This compliant solution is subject to the vagaries of the thread scheduler, which might schedule the tasks suboptimally. However, it avoids thread-starvation deadlock.


h2. Risk Assessment

Executing interdependent tasks in a thread pool can lead to denial of service.

|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| TPS01-J | low | probable | medium | {color:green}{*}P4{*}{color} | {color:green}{*}L3{*}{color} |


h2. Bibliography

| \[[API 2006|AA. References#API 06]\] | |
| \[[Gafter 2006|AA. References#Gafter 06]\] | [A Thread Pool Puzzler|http://gafter.blogspot.com/2006/11/thread-pool-puzzler.html] |
| \[[Goetz 2006|AA. References#Goetz 06]\] | 8.3.2, Managing queued tasks; 8.3.3, Saturation Policies; 5.3.3, Dequeues and work stealing |

----
[!The CERT Oracle Secure Coding Standard for Java^button_arrow_left.png!|TPS00-J. Use thread pools to enable graceful degradation of service during traffic bursts]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Oracle Secure Coding Standard for Java^button_arrow_up.png!|10. Thread Pools (TPS)]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Oracle Secure Coding Standard for Java^button_arrow_right.png!|TPS02-J. Ensure that tasks submitted to a thread pool are interruptible]

input and return
    return input;
  }
}

public final class BrowserManager {
  private final ExecutorService pool = Executors.newFixedThreadPool(10);
  private final int numberOfTimes;
  private static AtomicInteger count = new AtomicInteger(); // count = 0

  public BrowserManager(int n) {
    numberOfTimes = n;
  }

  public void perUser() {
    methodInvoker(numberOfTimes, "perProfile");
    pool.shutdown();
  }

  public void perProfile() {
    methodInvoker(numberOfTimes, "perTab");
  }

  public void perTab() {
    methodInvoker(numberOfTimes, "doSomething");
  }

  public void doSomething() {
    System.out.println(count.getAndIncrement());
  }

  public void methodInvoker(int n, final String method) {
    final BrowserManager manager = this;
    Callable<Object> callable = new Callable<Object>() {
      @Override public Object call() throws Exception {
        Method meth = manager.getClass().getMethod(method);
        return meth.invoke(manager);
      }
    };

    Collection<Callable<Object>> collection = 
        Collections.nCopies(n, callable);
    try {
      Collection<Future<Object>> futures = pool.invokeAll(collection);
    } catch (InterruptedException e) {
      // Forward to handler
      Thread.currentThread().interrupt(); // Reset interrupted status
    }
    // ...
  }

  public static void main(String[] args) {
    BrowserManager manager = new BrowserManager(5);
    manager.perUser();
  }
}

public final class BrowserManager {
  private final static ThreadPoolExecutor pool =
      new ThreadPoolExecutor(0, 10, 60L, TimeUnit.SECONDS,
                             new SynchronousQueue<Runnable>());
  private final int numberOfTimes;
  private static AtomicInteger count = new AtomicInteger(); // count = 0

  static {
    pool.setRejectedExecutionHandler(
    new ThreadPoolExecutor.CallerRunsPolicy());
  }

  // ...
}