Dispose pattern

"Dispose" redirects here. For other uses, see Disposal (disambiguation).

In computer programming, the dispose pattern is a design pattern which is used to handle resource cleanup and prevent resource leaks in runtime environments that use automatic garbage collection. The fundamental problem that the dispose pattern aims to solve is that, because objects in a garbage-collected environment have finalizers rather than destructors, there is no guarantee that an object will be destroyed at any deterministic point in time. The dispose pattern works around this by giving an object a method (usually called Dispose or similar) which frees any resources the object is holding onto.

Many garbage-collected languages offer language constructs to avoid having to call the dispose method explicitly in many situations. These language constructs leads to results similar to what is obtained with the Resource Acquisition Is Initialization (RAII) idiom in languages with deterministic memory management (e.g. C++).

Motivation

It is very common to write code similar to the listing below when using resources that might throw exceptions in garbage-collected languages:

Resource resource = null;
try {
    // Attempt to acquire the resource.
    resource = getResource();
    // Perform actions with the resource.
    ...
} finally {
   // Resource might not have been acquired, or already freed
    if (resource != null)
        resource.dispose();
}

The try...finally construct is necessary for proper exception safety, since the finally block enables execution of cleanup logic regardless of if an exception is thrown or not in the try block.

One disadvantage of this approach is that it requires the programmer to explicitly add cleanup code in a finally block. This leads to code size bloat, and failure to do so will lead to resource leakage in the program.

Language constructs

To make the dispose pattern less verbose, several languages have some kind of built-in support for resources held and released in the same block of code.

The C# language features the using statement that automatically calls the Dispose method on an object that implements the IDisposable interface:

using (Resource resource = GetResource())
{
    // Perform actions with the resource.
    ...
}

which is equal to:

Resource resource = GetResource()
try 
{
    // Perform actions with the resource.
    ...
}
finally 
{
    // Resource might not been acquired, or already freed
    if (resource != null) 
        ((IDisposable)resource).Dispose(); 
}

Similarly, the Python language has a with statement that can be used to similar effect with a context manager object. The context manager protocol requires implementing __enter__ and __exit__ methods which get automatically called by the with statement construct, to prevent duplication of code that would otherwise occur with the try/finally pattern.

with resource_context_manager() as resource:
    # Perform actions with the resource.
    ...
# Perform other actions where the resource is guaranteed to be deallocated.
...

The Java language introduced a new syntax called try-with-resources in Java version 7. It can be used on objects that implement the AutoCloseable interface (that defines method close()):

try ( OutputStream x = new OutputStream(...) ){
    //do something with x
} catch(IOException ex){
    //handle exception

  // The resource x is automatically closed
} // try

See also

• Object lifetime
• Resource Acquisition Is Initialization (RAII)