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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. See the guideline MET18-J. Do not use finalizers for additional reasons to avoid the use of finalizers.

The Java garbage collector is called to free unreferenced but as-yet unreleased memory. However, the Java garbage collector cannot free non-memory nonmemory resources such as open file descriptors and database connections. Consequently, programs that fail failing to release such non-memory resources can prematurely exhaust their pool of such resourceslead to resource exhaustion attacks. In addition, programs can experience resource starvation while waiting for finalize() a finalizer to release resources such as Lock or Semaphore objects. This can occur because Java lacks any temporal guarantee of when finalize() methods will finalizers execute , other than "sometime before program termination." Finally, output streams may cache object references; such cached objects will are not be 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 systemsAlso note that on the Windows platform, attempts to delete open files fail silently . See guideline FIO07(see FIO03-J. Do not create Remove temporary files in shared directoriesbefore termination for more information.

See also the related locking guideline LCK08-J. Ensure actively held locks are released on exceptional conditions.

).

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 includedResources other than raw memory must be explicitly freed in non-finalizer methods, due to the unsuitability of using finalizers.

Noncompliant Code Example (SQL Connection)

The problem of resource pool exhaustion is aggravated 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) { /* forwardForward to handler */ }
}

Noncompliant Code Example

This noncompliant code example attempts to address exhaustion of database connections by adding clean-up cleanup code in a finally block. However, either or both of rs and , 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) { 
  // forwardForward 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() could might throw an a SQLException; consequently. Consequently, stmtconn.close() would is never be calledcalled, 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) { 
  // forwardForward 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) { 
  // forwardForward to handler 
} finally {
  try {
    if (rs != null) {
      rs.close();
    }
  } finally {
    try {
      if (stmt != nullcatch (SQLException e) {
    // Forward   stmt.close();
      }to handler
    } finally {
      conn.close();try {
    }
  }
}

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)) {
  if (stmt != null) {stmt.close();}
    processResults(rs);
} catch (SQLException e) {
 
     // forwardForward to handler 
}

Noncompliant Code Example (File Handle)

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

public int processFile(String fileName) throws IOException,} FileNotFoundExceptionfinally {
   FileInputStream stream = new FileInputStream(fileName);try {
  BufferedReader bufRead = new BufferedReader(new InputStreamReader(stream));
  String line;
  while ((line = bufRead.readLine()) if (conn != null) {
    sendLine(lineconn.close();
  }
  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 it was created in the first place).

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

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

Compliant Solution (

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try-with-resources)

This compliant solution uses the try-with-resources statementconstruct, introduced in Java 1.SE 7, to release all acquired resources, regardless of any exceptions that might occur.ensure that resources are released as required:

try (FileInputStreamConnection streamconn = new FileInputStreamgetConnection(fileName);
     BufferedReaderStatement bufReadstmt = new BufferedReader(new InputStreamReader(stream))) {

  String line;
  while ((line = bufRead.readLine()) != null) {
    sendLine(line);
  }conn.createStatement();
     ResultSet rs = stmt.executeQuery(sqlQuery)) {
  processResults(rs);
} catch (IOExceptionSQLException e) { 
  // forwardForward 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 non-memory nonmemory system resources when they are no longer needed can result in resource exhaustion.

Automated Detection

Although sound automated detection of this 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 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.

Related Vulnerabilities

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

Other Languages

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

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Related Guidelines

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\[[API 2006|AA. Bibliography#API 06]\] [Class Object| http://java.sun.com/javase/6/docs/api/java/lang/Object.html]
\[[Goetz 2006b|AA. Bibliography#Goetz 06b]\]
\[[J2SE 2011|AA. Bibliography#J2SE 11]\] The try-with-resources Statement
\[[MITRE 2009|AA. Bibliography#MITRE 09]\] [CWE-405|http://cwe.mitre.org/data/definitions/405.html] "Asymmetric Resource Consumption (Amplification)", [CWE-404|http://cwe.mitre.org/data/definitions/404.html] "Improper Resource Shutdown or Release", [CWE-459 |http://cwe.mitre.org/data/definitions/459.html] "Incomplete Cleanup," [CWE-770|http://cwe.mitre.org/data/definitions/770.html], "Allocation of Resources Without Limits or Throttling"

Android Implementation Details

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

Bibliography

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Image Added Image Added Image AddedFIO05-J. Do not create multiple buffered wrappers on a single InputStream      12. Input Output (FIO)      FIO07-J. Do not create temporary files in shared directories