Java's garbage collection cannot be relied on to clean up everything. Specifically, connections, streams, files and other classes that implement the Closeable interface or it's super-interface, AutoCloseable, must be manually closed after creation. Further, that close call must be made in a finally block, otherwise an exception could keep the call from being made.

Failure to properly close resources will result in a resource leak which could bring first the application and then perhaps the box it's on to their knees.

Noncompliant Code Example

private void readTheFile() throws IOException {
  Path path = Paths.get(this.fileName);
  BufferedReader reader = Files.newBufferedReader(path, this.charset)) {
  // ...
  reader.close();  // Noncompliant
}

private void doSomething() {
  OutputStream stream = null;
  try{
    for (String property : propertyList) {
      stream = new FileOutputStream("myfile.txt");  // Noncompliant
      // ...
    }
  }catch(Exception e){
    // ...
  }finally{
    stream.close();  // Multiple streams were opened. Only the last is closed.
  }
}

Compliant Solution

private void readTheFile() throws IOException {
  Path path = Paths.get(this.fileName);
  BufferedReader reader = null;
  try {
    reader = Files.newBufferedReader(path, this.charset)) {
    // ...
  } finally {
    if (reader != null) {
      reader.close();
    }
  }
}

private void doSomething() {
  OutputStream stream = null;
  try{
    stream = new FileOutputStream("myfile.txt");
    for (String property : propertyList) {
      // ...
    }
  }catch(Exception e){
    // ...
  }finally{
    stream.close();
  }
}

Exceptions

Java 7 introduced the try-with-resources statement, which implicitly closes Closeables. All resources opened in a try-with-resources statement are ignored by this rule. 

try (BufferedReader br = new BufferedReader(new FileReader(fileName))) {
  //...
}
catch ( ... ) {
  //...
}

See