Avoid Injecting Closable Resources

Avoid Injecting Closable Resources

If you provide Closable resources through dependency injection, it can be difficult to effectively manage the lifecycle of the Closable:

class MyModule extends AbstractModule() {
  @Provides
  FileOutputStream provideFileStream() {
    return new FileOutputStream("/tmp/outfile");
  }
}

...

class Client {
  private final FileOutputStream fos;
  @Inject Client(FileOutputStream fos, OtherDependency other, ...) {
     this.fos = fos;
  }
  void doSomething() throws IOException {
    fos.write("hello!");
  }
}

There are a number of issues with this approach as it relates to resource management:

• If multiple Client classes are constructed, multiple output streams are opened against the same file, and writes to the file may clash with each other.

• It's not clear which class has the responsibility of closing the FileOutputStream resource:

  • If the resource is created for the sole ownership of Client, then it makes sense for the client to close it.
  • However, if the resource is scoped (e.g.: you add @Singleton to the @Provides method), then suddenly it's not the responsibility of Client to close the resource. If one Client closes the stream, then all of the other users of that stream will be dealing with a closed resource. There needs to be some other 'resource manager' object that also gets that stream, and its closing functions are call in the right place. That can be tricky to do correctly.

• If, for example, the construction of the other dependencies of Client fails (resulting in a ProvisionException), then the FileOutputStream that may have been constructed leaks and isn't properly closed, even if Client normally closes its resources correctly.

The preferred solution is to not inject closable resources, but instead, objects that can expose short-lived closable resources that are used as necessary. The following example uses Guava's CharSource as the resource manager object:

class MyModule extends AbstractModule() {
  @Provides
  CharSink provideCharSink() {
    return Files.asCharSink(new File("/tmp/outfile"), StandardCharsets.UTF_8);
  }
}

...

class Client {
  private final CharSink sink;
  @Inject Client(CharSink sink, OtherDependency other, ...) {
     this.sink = sink;
  }
  void doSomething() throws IOException {
    sink.write("hello!"); // Opens the file at this point, and closes once its done.
  }
}

If there's not a similar non-closable resource, you can write a simple wrapper:

class ResourceManager {
  @Inject ResourceManager(@Config String configs, ...) {}

  /**
   * Returns a new thing for you to use and dispose of
   */
  OutputStream provideInstance() { return new...(); }
}

...
class Client {
  private final ResourceManager resource;
  @Inject Client(ResourceManager resource, OtherDependency other, ...) {
     this.resource = resource;
  }
  void doSomething() {
    try (OutputStream actualStream = resource.provideInstance()) {
      // write to actualStream, closing with try-with-resources
    }
  }
}

This pattern can be extended to other resources: as opposed to injecting database connection handles directly, inject connection pool objects that require your object to ask for those connection objects when they're needed and close them safely.