EXC31-J. Handle checked exceptions that can be thrown within a finally block

It is possible that an exception gets thrown in the finally block even though it escapes detection at compile time. This can prevent other clean-up statements from getting executed.

Noncompliant Code Example

The finally clause closes the reader object in this noncompliant example. However, it is incorrectly assumed that the statements within the finally block cannot throw exceptions. Notably, close() can throw an IOException which in turn prevents any subsequent clean-up lines from getting executed. This is not detected at compile time since close() throws the same exception type as read or write.

public class Login {
  static void checkPassword(String password_file) throws IOException {
  
    StringBuffer fileData = new StringBuffer(1000);
    BufferedReader reader = new BufferedReader(new FileReader(password_file));

    try {
          int n;
          char[] passwd = new char[1024];
          while ((n = reader.read(passwd)) >= 0) {
            String readData = String.valueOf(passwd, 0, n);
            fileData.append(readData);
            passwd = new char[1024];
          }
          String realPassword = "javac<at:var at:name="f3b" />b3";
          System.out.println(fileData.toString());
        
          if (fileData.toString().equals(realPassword)) {
            System.out.println("Login successful");
          }
          else {
            System.out.println("Login failed");
          }
    } finally {
      reader.close();
      //other clean-up code 
    }
}

  public static void main(String[] args) throws IOException {
    String path = "c:\\password.txt";
    checkPassword(path);
  }
}

Compliant Solution

This compliant solution correctly places the close() statement in a try-catch block. As a result an IOException can be handled without letting it propagate any further.

public class Login {
  static void checkPassword(String password_file) throws IOException {
  
    StringBuffer fileData = new StringBuffer(1000);
    BufferedReader reader = new BufferedReader(new FileReader(password_file));

    try {
          int n;
          char[] passwd = new char[1024];
          while ((n = reader.read(passwd)) >= 0) {
            String readData = String.valueOf(passwd, 0, n);
            fileData.append(readData);
            passwd = new char[1024];
          }
          String realPassword = "javac<at:var at:name="f3b" />b3";
          System.out.println(fileData.toString());
        
          if (fileData.toString().equals(realPassword)) {
            System.out.println("Login successful");
          }
          else {
            System.out.println("Login failed");
          }
    } finally {
      try {     //enclose in try-catch block
        reader.close();
      }catch (IOException ie) {ie.getMessage();}
      //other clean-up code
    }
}

  public static void main(String[] args) throws IOException {
    String path = "c:\\password.txt";
    checkPassword(path);
  }
}

Compliant Solution 2

If the need to close a stream without throwing an exception occurs often, then an alternative solution to wrapping every call of close() in its own try-catch block is to write a method, as shown in this compliant solution.

public class Login {
  static void checkPassword(String password_file) throws IOException {
  
    StringBuffer fileData = new StringBuffer(1000);
    BufferedReader reader = new BufferedReader(new FileReader(password_file));

    try {
          int n;
          char[] passwd = new char[1024];
          while ((n = reader.read(passwd)) >= 0) {
            String readData = String.valueOf(passwd, 0, n);
            fileData.append(readData);
            passwd = new char[1024];
          }
          String realPassword = "javac<at:var at:name="f3b" />b3";
          System.out.println(fileData.toString());
        
          if (fileData.toString().equals(realPassword)) {
            System.out.println("Login successful");
          }
          else {
            System.out.println("Login failed");
          }
    } finally {
          closeIgnoringException(reader);
          //other clean-up code 
    }
}

  private static void closeIgnoringException(Closeable s) {
    if (s != null) {
      try {
        s.close();
      } catch (IOException ie) {
        // Ignore exception if close fails
      }
    }
  }

  public static void main(String[] args) throws IOException {
    String path = "c:\\password.txt";
    checkPassword(path);
  }
}

Risk Assessment

Failing to handle an exception in a finally block can lead to unexpected results.

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
EXC31-J	low	unlikely	medium	P2	L3

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[Bloch 05]] Puzzle 41: Field and Stream
[[Harold 99]]
[[Chess 07]] 8.3 Preventing Resource Leaks (Java)

EXC30-J. Do not exit abruptly from a finally block 11. Exceptional Behavior (EXC) 49. Miscellaneous (MSC)

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