Connections, streams, files, and other classes that implement the Closeable interface or its super-interface,
AutoCloseable, needs to be closed after use. Further, that close call must be made in a finally block otherwise
an exception could keep the call from being made. Preferably, when class implements AutoCloseable, resource should be created using
"try-with-resources" pattern and will be closed automatically.
Failure to properly close resources will result in a resource leak which could bring first the application and then perhaps the box the application
is 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
// ...
Files.lines("input.txt").forEach(System.out::println); // Noncompliant: The stream needs to be closed
}
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(String fileName) throws IOException {
Path path = Paths.get(fileName);
try (BufferedReader reader = Files.newBufferedReader(path, StandardCharsets.UTF_8)) {
reader.readLine();
// ...
}
// ..
try (Stream<String> input = Files.lines("input.txt")) {
input.forEach(System.out::println);
}
}
private void doSomething() {
OutputStream stream = null;
try {
stream = new FileOutputStream("myfile.txt");
for (String property : propertyList) {
// ...
}
} catch (Exception e) {
// ...
} finally {
stream.close();
}
}
Exceptions
Instances of the following classes are ignored by this rule because close has no effect:
-
java.io.ByteArrayOutputStream
-
java.io.ByteArrayInputStream
-
java.io.CharArrayReader
-
java.io.CharArrayWriter
-
java.io.StringReader
-
java.io.StringWriter
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 ( ... ) {
//...
}
