FIO02-J. Detect and handle file-related errors

Java's file-manipulation methods often indicate failure with a return value instead of throwing an exception. The Java Tutorials for Java 7 note:

Prior to the Java SE 7 release, the java.io.File class was the mechanism used for file I/O, but it had several drawbacks.

One of these drawbacks is that:

Many methods didn't throw exceptions when they failed, so it was impossible to obtain a useful error message. For example, if a file deletion failed, the program would receive a "delete fail" but wouldn't know if it was because the file didn't exist, the user didn't have permissions, or there was some other problem.

Consequently, programs that ignore the return values from file operations often fail to detect that those operations have failed. Java programs must check the return values of methods that perform file I/O (this is a specific instance of rule EXP00-J. Do not ignore values returned by methods).

Noncompliant Code Example (delete())

This noncompliant code example attempts to delete a specified file but gives no indication of its success. The Java Platform, Standard Edition 6 API Specification [[API 2006]] requires File.delete() to throw a SecurityException only when the program lacks authorization to delete the file. No other exceptions are thrown, so the deletion can silently fail.

File file = new File(args[0]);
file.delete();

Compliant Solution

This compliant solution checks the return value of delete().

File file = new File("file");
if (!file.delete()) {
  System.out.println("Deletion failed");
}

Compliant Solution (Java SE 7)

This compliant solution uses the java.nio.file.Files.delete() method from Java SE 7 to delete the file.

Path file = new File(args[0]).toPath();
try {
  Files.delete(file);
} catch (IOException x) {
  System.out.println("Deletion failed");
  // handle error
}

The Java SE 7 Documentation [[J2SE 2011]] defines Files.delete() to throw the following exceptions:

Since SecurityException is a runtime exception, it need not be declared. And NoSuchFileException and DirectoryNotExmptyException both inherit from IOException, they will be caught by the compliant solution's catch clause.

Risk Assessment

Failure to check the return values of methods that perform file I/O can result in unexpected behavior.

Related Guidelines

Bibliography

      12. Input Output (FIO)