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Methods invoked from within a finally block can throw an exception. Failing Failure to catch and handle such exceptions results in the abrupt termination of the entire try block, suppressing . Abrupt termination causes any exception thrown in the try block to be lost, preventing any possible recovery method from handling that specific problem. Additionally, the transfer of control associated with the exception prevents may prevent execution of any expressions or statement statements that occurs occur after the point in the finally block from which the exception is thrown. Consequently, programs must appropriately handle checked exceptions that are thrown from within a finally block.

Allowing checked exceptions to escape a finally block also violates ERR04-J. Do not complete abruptly from a finally block.

Noncompliant Code Example

This noncompliant code example contains a finally block that closes the reader object. The programmer incorrectly assumes that the statements in the finally block cannot throw exceptions , and consequently fails to appropriately handle the any exception appropriatelythat may arise.

public class Operation {
  privatepublic static void doOperation(String some_file) throws IOException {
    // ... Code to check or set character encoding ...
    try {
      BufferedReader reader =
          new BufferedReader(new FileReader(some_file));
      try {
        // Do operations 
      } finally {
        reader.close();
        // ... Other clean-upcleanup code ...
    }
  }

   public static} voidcatch main(String[]IOException argsx) throws{
 IOException {
    String// pathForward = "somepath";to handler
    doOperation(path);}
  }
}

The close() method can throw an IOException, which, if thrown, would prevent execution of any subsequent clean-up cleanup statements. The compiler This problem will not diagnose this problem because the doOperation() method explicitly declares that it may throw IOExceptionbe diagnosed by the compiler because any IOException would be caught by the outer catch block. Also, an exception thrown from the close() operation can mask any exception that gets thrown during execution of the Do operations block, preventing proper recovery.

Compliant Solution (Handle Exceptions in finally Block)

This compliant solution encloses the close() method invocation in a try-catch block of its own within the finally block. Consequently, an the potential IOException can be handled without permitting allowing it to propagate fartherfurther.

public class Operation {
  public static void doOperation(String some_file) throws IOException {
    BufferedReader reader = new BufferedReader(new FileReader(some_file));

    try {
      // Do operations
    } finally {
  ... Code to check or set character encoding ...
    try {    
      BufferedReader reader //=
 Enclose in try-catch block
      new  reader.close()BufferedReader(new FileReader(some_file));
      } catch (IOException ie) try {
        // ForwardDo tooperations handler
      }
      // Other clean-up codefinally {
    }
  }

  public static void main(String[] args) throws IOException try {
    String path = "somepath";
    doOperationreader.close(path);
  }
}

While suppressing a caught exception normally violates ERR00-J. Do not suppress or ignore checked exceptions, this particular code is permitted under ERR00-EX0, as the reader is never accessed again, so an error in closing it does not affect future program behavior.

Compliant Solution (Dedicated Method to Handle Exceptions)

When closing a stream without throwing an exception is a frequent pattern in the code, an alternative solution is to use a closeHandlingException() method, as shown in this compliant solution.

public class Operation {
  static void doOperation(String some_file) throws IOException       } catch (IOException ie) {
    BufferedReader reader = new BufferedReader(new FileReader(some_file));

    try { // Forward to handler
      // Do operations
    } finally {
      closeHandlingException(reader);
      // ... Other clean-upcleanup code ...
    }
  } 

  private static void closeHandlingException(BufferredReader s) {
    if (s != null) {
      try {
        s.close();
      } catch (IOException iex) {
        // Forward to handler
      }
    }
  }

  public static void main(String[] args) throws IOException {
    doOperation("somepath");
  }
}

Suppressing a caught exception is again permitted under exception ERR00-EX0 of ERR00-J. Do not suppress or ignore checked exceptions.

Compliant Solution (

...

try-with-resources)

Java 1.SE 7 provides introduced a new feature , called try-with-resources, that  that can close certain resources automatically should in the event of an error occur. This compliant solution uses try-with-resources to properly close the file.

public class Operation {
  public static void doOperation(String some_file) throws IOException {
    try (BufferedReader reader = new BufferedReader(new FileReader(some_file))) {
      // Do operations
    }
  }

  public static void main(String[] args) {... Code to check or set character encoding ...
    try ( // try-with-resources
    if (args.length <BufferedReader 1)reader {=
      System.out.println("Please supply a path asnew an argument");
      return;BufferedReader(new FileReader(some_file))) {
    }
  // Do try {operations
      doOperation( args[0]);
    } catch (IOException ex) {
      System.outerr.println("thrown exception: " + ex.toString());
      Throwable[] suppressed = ex.getSuppressed();
      for (int i = 0; i < suppressed.length; i++) {
        System.outerr.println("suppressed exception: " 
            + suppressed[i].toString());
      }

      // how handle exception(s) Forward to handler
    }
  }

  public static void main(String[] args) {
    if (args.length < 1) {
      System.out.println("Please supply a path as an argument");
      return;
    }
    doOperation(args[0]);
  }
}

If When an error IOException occurs in the try block of the doOperation() method it will propagate out of the method and be , it is caught by the catch block and printed as the thrown exception. If an error Exceptions that occur while creating the BufferedReader are included. When an IOException occurs while closing the reader, that error will propagate out of doOperation() and be exception is also caught by the catch block and printed as the thrown exception. If both errors occur, the try-block error will propagates out of the doOperation() and be printed block and closing the reader throw an IOException, the catch clause catches both exceptions and prints the try block exception as the thrown exception. The close error exception is suppressed and printed as the suppressed exception. In all cases, the reader is safely closed.

...

Failure to handle an exception in a finally block can lead to may have unexpected results.

Automated Detection

Related Vulnerabilities

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

Bibliography

\[[Bloch 2005|AA. Bibliography#Bloch 05]\] Puzzle 41: Field and Stream
\[[Chess 2007|AA. Bibliography#Chess 07]\] 8.3 Preventing Resource Leaks (Java)
\[[Harold 1999|AA. Bibliography#Harold 99]\]
\[[J2SE 2011|AA. Bibliography#J2SE 11]\] The try-with-resources Statement

Related Guidelines

Bibliography

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Image Added Image Added Image AddedERR04-J. Do not exit abruptly from a finally block      06. Exceptional Behavior (ERR)      ERR06-J. Do not allow exceptions to expose sensitive information