FIO04-J. Close resources when they are no longer needed

The Java garbage collector is called to free unreferenced but as-yet unreleased memory. However, the Java garbage collector cannot free non-memory resources such as file descriptors and database connections. Consequently, failing to release such non-memory resources can lead to resource exhaustion attacks. In addition, programs can experience resource starvation while waiting for finalize() to release resources such as Lock or Semaphore objects. This can occur because Java lacks any temporal guarantee of when finalize() methods execute, other than "sometime before program termination." Finally, output streams may cache object references; such cached objects are not garbage collected until after the output stream is closed. Consequently, output streams should be closed promptly after use.

A program may leak resources when it relies on finalize() to release system resources or when there is confusion over which part of the program is responsible for releasing system resources. In a busy system, the delay before the finalize() method is called for an object provides a window of vulnerability during which an attacker could induce a denial-of-service attack. Consequently, resources other than raw memory must be explicitly freed in non-finalizer methods, because of the unsuitability of using finalizers. See the rule MET12-J. Do not use finalizers for additional reasons to avoid the use of finalizers.

Note that on the Windows platform, attempts to delete open files fail silently. See rule FIO03-J. Remove temporary files before termination for more information.

Noncompliant Code Example (File Handle)

This noncompliant code example opens a file and uses it, but fails to explicitly close the file.

public int processFile(String fileName) throws IOException, FileNotFoundException {
  FileInputStream stream = new FileInputStream(fileName);
  BufferedReader bufRead = new BufferedReader(new InputStreamReader(stream));
  String line;
  while ((line = bufRead.readLine()) != null) {
    sendLine(line);
  }
  return 1;
}

Compliant Solution

This compliant solution releases all acquired resources, regardless of any exceptions that might occur. Even though dereferencing bufRead might result in an exception, the FileInputStream object is closed as required (if created in the first place).

try {
  final FileInputStream stream = new FileInputStream(fileName);
  try {
    final BufferedReader bufRead = new BufferedReader(new InputStreamReader(stream));
    String line;
    while ((line = bufRead.readLine()) != null) {
      sendLine(line);
    }
  } finally {
    if (stream != null) { 
      try {
        stream.close();
      } catch (IOException e) { 
        // forward to handler 
      }
    }
  }
} catch (IOException e) { 
  // forward to handler 
}

Compliant Solution (Java 1.7, try-with-resources)

This compliant solution uses the try-with-resources statement, introduced in Java 1.7, to release all acquired resources, regardless of any exceptions that might occur.

try (FileInputStream stream = new FileInputStream(fileName);
     BufferedReader bufRead = new BufferedReader(new InputStreamReader(stream))) {

  String line;
  while ((line = bufRead.readLine()) != null) {
    sendLine(line);
  }
} catch (IOException e) { 
  // forward to handler 
}

The try-with-resource construct sends any IOException to the catch clause, where it is forwarded to an exception handler. This includes exceptions generated during the allocation of resources (that is, the creation of the FileInputStream or BufferedReader. It also includes any IOException thrown during the while loop. Finally, it includes any IOException generated by closing bufRead or stream.

Noncompliant Code Example (SQL Connection)

The problem of resource pool exhaustion is exacerbated in the case of database connections. Many database servers allow only a fixed number of connections, depending on configuration and licensing. Consequently, failure to release database connections can result in rapid exhaustion of available connections. This noncompliant code example fails to close the connection when an error occurs during execution of the SQL statement or during processing of the results.

public void getResults(String sqlQuery) {
  try {
    Connection conn = getConnection();
    Statement stmt = conn.createStatement();
    ResultSet rs = stmt.executeQuery(sqlQuery);
    processResults(rs);
    stmt.close();
  } catch (SQLException e) { /* forward to handler */ }
}

Noncompliant Code Example

This noncompliant code example attempts to address exhaustion of database connections by adding clean-up code in a finally block. However, either or both of rs and stmt could be null, causing the code in the finally block to throw a NullPointerException.

Statement stmt = null;
ResultSet rs = null
Connection conn = getConnection();
try {
  stmt = conn.createStatement();
  rs = stmt.executeQuery(sqlQuery);
  processResults(rs);
} catch(SQLException e) { 
  // forward to handler  
} finally {
  rs.close();
  stmt.close();
}

Noncompliant Code Example

In this noncompliant code example, the call to rs.close() might throw a SQLException. Consequently, stmt.close() is never called. This is a violation of ERR05-J. Do not let checked exceptions escape from a finally block.

Statement stmt = null;
ResultSet rs = null;
Connection conn = getConnection();
try {
  stmt = conn.createStatement();
  rs = stmt.executeQuery(sqlQuery);
  processResults(rs);
} catch (SQLException e) { 
  // forward to handler 
} finally {
  if (rs != null) {
    rs.close();
  }
 
  if (stmt != null) {
    stmt.close();
  }
}

Compliant Solution

This compliant solution ensures that resources are released as required.

Statement stmt = null;
ResultSet rs = null;
Connection conn = getConnection();
try {
    stmt = conn.createStatement();
    rs = stmt.executeQuery(sqlQuery);
    processResults(rs);
} catch (SQLException e) { 
  // forward to handler 
} finally {
  if (rs != null) {
    try {
      rs.close();
    } catch (SQLException e) {
      // forward to handler 
    } finally {
      if (stmt != null) {
        try {
          stmt.close();
      } catch (SQLException e) {
        // forward to handler 
      } finally {
        if (conn != null) {
          try {
            conn.close();
          } catch (SQLException e) {
            // forward to handler 
          }
        }
      }
    }
  }
}

Compliant Solution (Java 1.7, try-with-resources)

This compliant solution uses the try-with-resource construct, introduced in Java 1.7, to ensure that resources are released as required.

try (Connection conn = getConnection();
     Statement stmt = conn.createStatement();
     ResultSet rs = stmt.executeQuery(sqlQuery)) {

    processResults(rs);
} catch (SQLException e) { 
  // forward to handler 
}

The try-with-resource construct sends any SQLException to the catch clause, where it gets forwarded to an exception handler. This includes exceptions generated during the allocation of resources (that is, the creation of the Connection, Statement, or ResultSet). It also includes any SQLException thrown by processResults(). Finally, it includes any SQLException generated by closing rs, stmt, or conn.

Risk Assessment

Failure to explicitly release non-memory system resources when they are no longer needed can result in resource exhaustion.

Automated Detection

Although sound automated detection of this vulnerability is not feasible in the general case, many interesting cases can be soundly detected.

The Coverity Prevent Version 5.0 RESOURCE_LEAK checker can detect instances where there is leak of a socket resource or leak of a stream representing a file or other system resources.

Related Vulnerabilities

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

Related Guidelines

Bibliography

FIO05-J. Do not create multiple buffered wrappers on a single InputStream      12. Input Output (FIO)