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The Java Development Kit 1.7 (JDK 1.7) introduced the "trythe try-with-resources " statement [JLS 2011] statement (see the JLS, §14.20.3, "try-with-resources" that makes it much easier to deal with  [JLS 2013]),  which simplifies correct use of resources that implement the java.lang.AutoCloseable interface, including those that implement the java.io.Closeable interface.

Using the try-with-resources statement avoids problems that can arise when closing resources with an ordinary try-catch-finally block, such as failing to close a resource because an exception is thrown as a result of closing another resource, or masking an important exception when a resource is closed.    

Use of the try-with-resources statement is also illustrated in in ERR05-J. Do not let checked exceptions escape from a finally block, FIO03-J. Remove temporary files before termination, and FIO04-J. Release resources when they are no longer needed.

Noncompliant Code Example

This noncompliant code example uses an ordinary try-catch-finally block to try in an attempt to close two resources.  However, if closing the BufferedReader br results in an exception being thrown, then the BufferedWriter bw will not be closed.  

public void processFile(String inPath, String outPath) 
    throws IOException{
	  BufferedReader br = null;
    BufferedWriter bw = null;
    try {
    	br = new BufferedReader(new FileReader(inPath));
        bw = new BufferedWriter(new FileWriter(outPath));
        // processProcess the input and produce the output
  } finally {
   } finallytry {
      	if (br != null) {
        	br.close();
        }
        if (bw != null) {
        	bw.close();
      }
    } catch (IOException x) {
      // Handle error
    }
  }
}

However, if an exception is thrown when the BufferedReader br is closed, then the BufferedWriter bw will not be closed.

Compliant Solution (finally block)

This compliant solution uses finally blocks a second finally block to guarantee that both br and bw are properly closed, regardless of any exceptions that may be thrown during the close operationsthat bw is properly closed even when an exception is thrown while closing br.

public void processFile(String inPath, String outPath)
   throws throws IOException {
  BufferedReader br = null;
  BufferedWriter bw = null;
  try {
    br = new BufferedReader(new FileReader(inPath));
    bw = new BufferedWriter(new FileWriter(outPath));
    // ... processProcess the input and produce the output
  } finally {
    if (br != null) {
      try {
        br.close();
      } catch (IOException x) {
        // handleHandle error
      } finally {
        if (bw != null) {
          try {
            bw.close();
          } catch (IOException x) {
            // handleHandle error
          }
        }
      }
    }
  }
}

Compliant Solution (try-with-resources)

This compliant solution uses a try-with-resources statement which will guarantee that to manage both br and bw are closed, regardless of any exceptions potentially thrown during the close operations..  

public void processFile(String inPath, String outPath)
 throws  throws IOException{
	  try (BufferedReader br = new BufferedReader(new FileReader(inPath));
         BufferedWriter bw = new BufferedWriter(new FileWriter(outPath));) {
            // process the input and produce the output
        }
}

Noncompliant Code Example

This noncompliant code example uses an ordinary try-finally block to try to close a resource.  However, if there is an exception thrown during the processing of the input and another exception thrown when closing the Bufferedreader br, then the exception thrown as a result of processing the input will be lost, and important information about that exceptional circumstance may be missed.

public void processFile(String inPath) throws IOException{
  BufferedReader br = null;
  try {
    br = new BufferedReader(new FileReader(inPath));
    // process Process the input and produce the output
    } finally {
      if (br != null) {
        br.close();
      }
    }
  }
}

Compliant Solution (try-with-resources)

This compliant solution uses a try-with-resources statement which will not suppress any exceptions thrown during the processing of the input while still guaranteeing that br is closed. It demonstrates how to access every exception that may be produced from the try-with-resources block.

public void processFile(String inPath) throws IOException{
  try (BufferedReader br = new BufferedReader(new FileReader(inPath));) {
    // process the input and produce the output
  } catch (IOException ex) {
    System.err.println("thrown exception: " + ex.toString());
    Throwable[] suppressed = ex.getSuppressed();
    for (int i = 0; i < suppressed.length; i++) {
      System.err.println("suppressed exception: " + suppressed[i].toString());
    }
  }
}

This solution preserves any exceptions thrown during the processing of the input while still guaranteeing that both br and bw are properly closed, regardless of what exceptions occur. Finally, this code demonstrates how to access every exception that may be produced from the try-with-resources block.

If only one exception is thrown, either during opening, processing, or closing of the filefiles, it the exception will be printed by the after "thrown exception:" statement. If an exception is thrown during processing, and another one a second exception is thrown while trying to close the either file, then the first exception will be printed after "thrown exception:" statement will print the exception encountered while closing the file, and the second exception will be printed after "suppressed exception:" statement will print the exception encountered during processing.

Applicability

Failing to use a try-with-resources statement when dealing correctly handle all failure cases when working with closeable resources may result in some resources not being closed , or in important exceptions being masked, possibly resulting in a denial of service attack.. Note that failure to use a try-with-resources statement cannot be considered a security vulnerability in and of itself because it is possible to write a correctly structured group of nested try-catch-finally blocks guarding the resources that are in use (see ERR05-J. Do not let checked exceptions escape from a finally block). That said, failure to correctly handle such error cases is a common source of vulnerabilities. Use of a try-with-resources statement mitigates this issue by guaranteeing that the resources are managed correctly and that exceptions are never masked.

Automated Detection

Bibliography

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