The Java garbage collector is called to free unreferenced but as-yet unreleased memory. However, the garbage collector cannot free nonmemory resources such as open file descriptors and database connections. Consequently, failing to release such resources can lead to resource exhaustion attacks. In addition, programs can experience resource starvation while waiting for a finalizer to release resources such as Lock or Semaphore objects. This can occur because Java lacks any temporal guarantee of when finalizers 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 finalizers 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 finalizer is called for an object provides a window of vulnerability during which an attacker could induce a denial-of-service (DoS) attack. Consequently, resources other than raw memory must be explicitly freed in nonfinalizer methods because of the unsuitability of using finalizers. See MET12-J. Do not use finalizers for additional reasons to avoid the use of finalizers.

Note that on Windows systems, attempts to delete open files fail silently (see 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.

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 (try-with-resources)

This compliant solution uses the try-with-resources statement, introduced in Java SE 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-resources construct sends any IOException to the catch clause, where it is forwarded to an exception handler. Exceptions generated during the allocation of resources (that is, the creation of the FileInputStream or BufferedReader), as well as any IOException thrown during execution of the while loop and any IOException generated by closing bufRead or stream, are included.

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(); conn.close();
  } catch (SQLException e) { /* Forward to handler */ }
}

Noncompliant Code Example

This noncompliant code example attempts to address exhaustion of database connections by adding cleanup code in a finally block. However, rs, stmt, or conn 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(); conn.close();
}

Noncompliant Code Example

In this noncompliant code example, the call to rs.close() or the call to stmt.close() might throw a SQLException. Consequently, conn.close() is never called, which violates 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();
  } if (conn !=null) {
       conn.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();}
  } catch (SQLException e) {
    // Forward to handler
  } finally {
    try {
      if (stmt != null) {stmt.close();}
    } catch (SQLException e) {
      // Forward to handler
    } finally {
      try {
        if (conn != null) {conn.close();}
      } catch (SQLException e) {
        // Forward to handler
      }
    }
  }
}

Compliant Solution (try-with-resources)

This compliant solution uses the try-with-resources construct, introduced in Java SE 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-resources construct sends any SQLException to the catch clause, where it is forwarded to an exception handler. Exceptions generated during the allocation of resources (that is, the creation of the Connection, Statement, or ResultSet), as well as any SQLException thrown by processResults() and any SQLException generated by closing rs, stmt, or conn are included.

Risk Assessment

Failure to explicitly release nonmemory 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.

Some static analysis tools can detect cases in which there is leak of a socket resource or leak of a stream representing a file or other system resources.

Tool	Version	Checker	Description
CodeSonar		JAVA.ALLOC.LEAK.NOTCLOSED JAVA.ALLOC.LEAK.NOTSTORED	Closeable Not Closed (Java) Closeable Not Stored (Java)
Coverity	7.5	ITERATOR JDBC_CONNECTION RESOURCE_LEAK	Implemented
Parasoft Jtest		CERT.FIO04.LEAKS CERT.FIO04.CIO CERT.FIO04.CCR	Ensure resources are deallocated Close input and output resources in "finally" blocks Close all "java.io.Closeable" resources in a "finally" block
SonarQube		S2095	Implemented

Related Guidelines

Android Implementation Details

The compliant solution (try-with-resources) is not yet supported at API level 18 (Android 4.3).

Bibliography