FIO32-J. Ensure all resources are properly closed when they are no longer needed

If a program relies on finalize() to release system resources, or if there is confusion over which part of the program is responsible for releasing system resources, then there exists a possibility of a potential resource leak. In a busy system, there might be a time gap before the finalize() method is called for an object. An attacker might exploit this vulnerability to induce a denial-of-service attack. The guideline OBJ02-J. Avoid using finalizers has more information on the demerits of using finalizers.

The Java garbage collector is called to free up unreleased memory. However, if the program relies on non-memory resources like file descriptors and database connections, unreleased resources might lead the program to prematurely exhaust its pool of resources. In addition, if the program uses resources like Lock or Semaphore, waiting for finalize() to release the resources may result in resource starvation. Caching of object references in the output stream also implies that the objects will not be garbage collected unless the streams are closed promptly after use.

Also note that on the Windows platform, attempts to delete open files fail silently. See FIO34-J. Do not create temporary files in shared directories for more information.

Noncompliant Code Example

The problem of resource pool exhaustion is aggravated in the case of database connections. Traditionally, database servers allow a fixed number of connections, depending on configuration and licensing. Failing to release database connections can result in rapid exhaustion of available connections. This noncompliant code example does not close the connection if an error occurs while executing the statement or while processing 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

While being slightly better than the previous noncompliant code example, this code is also noncompliant. Both rs and stmt might be null and the clean-up code in the finally block may result in a NullPointerException.

Statement stmt = null;
ResultSet rs = null
Connection conn = getConnection(0;
try {
  stmt = conn.createStatement();
  rs = stmt.executeQuery(sqlQuery);
  processResults(rs);
} catch(SQLException e) { 
  // forward to handler  
} finally {
  rs.close();
  stmt.close();
}

Noncompliant Code Example

Again, while being still better, this code snippet is still noncompliant. This is because rs.close() might itself result in a SQLException, and so stmt.close() will never be called.

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 {
  try {
    if(rs != null) {
      rs.close();
    }
  } finally {
    try {
      if(stmt != null) {
        stmt.close();
      }
    }
    finally {
      conn.close();
    }
  }
}

Noncompliant Code Example

This noncompliant code example opens a file, uses it, but does not explicitly close the handle.

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 bufRead might result in an exception, if a FileInputStream object is instantiated, it will be closed as required.

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

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

Risk Assessment

Acquiring non-memory system resources and not releasing them explicitly may result in resource exhaustion.

Related Vulnerabilities

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

Other Languages

This rule appears in the C Secure Coding Standard as FIO42-C. Ensure files are properly closed when they are no longer needed.

This rule appears in the C++ Secure Coding Standard as FIO42-CPP. Ensure files are properly closed when they are no longer needed.

References

[[API 06]] Class Object
[[Goetz 06b]]
[[MITRE 09]] CWE ID 405 "Asymmetric Resource Consumption (Amplification)", CWE ID 404 "Improper Resource Shutdown or Release", CWE ID 459 "Incomplete Cleanup"

FIO31-J. Defensively copy mutable inputs and mutable internal components      09. Input Output (FIO)      FIO33-J. Exclude user input from format strings