Threads and tasks that block on operations involving network or file I/O must provide callers with an explicit termination mechanism to prevent DoS vulnerabilities.
h2. Noncompliant Code Example (Blocking I/O, Volatile Flag)
This noncompliant code example uses a volatile {{done}} flag to indicate that whether is safe to shut down the thread, as suggested in rule [THI05-J. Do not use Thread.stop() to terminate threads]. However, when the thread is blocked on network I/O as a consequence of invoking the {{readLine()}} method, it cannot respond to the newly-set flag until the network I/O is complete. Consequently, thread termination may be indefinitely delayed.
{code:bgColor=#FFcccc}
// Thread-safe class
public final class SocketReader implements Runnable {
private final Socket socket;
private final BufferedReader in;
private volatile boolean done = false;
private final Object lock = new Object();
public SocketReader(String host, int port) throws IOException {
this.socket = new Socket(host, port);
this.in = new BufferedReader(
new InputStreamReader(this.socket.getInputStream())
);
}
// Only one thread can use the socket at a particular time
@Override public void run() {
try {
synchronized (lock) {
readData();
}
} catch (IOException ie) {
// Forward to handler
}
}
public void readData() throws IOException {
String string;
while (!done && (string = in.readLine()) != null) {
// Blocks until end of stream (null)
}
}
public void shutdown() {
done = true;
}
public static void main(String[] args)
throws IOException, InterruptedException {
SocketReader reader = new SocketReader("somehost", 25);
Thread thread = new Thread(reader);
thread.start();
Thread.sleep(1000);
reader.shutdown(); // Shutdown the thread
}
}
{code}
h2. Noncompliant Code Example (Blocking I/O, Interruptible)
This noncompliant code example is similar to the preceding example but uses thread interruption to shut down the thread. Network I/O on a {{java.net.Socket}} is unresponsive to thread interruption.
{code:bgColor=#FFcccc}
// Thread-safe class
public final class SocketReader implements Runnable {
// other methods...
public void readData() throws IOException {
String string;
while (!Thread.interrupted() && (string = in.readLine()) != null) {
// Blocks until end of stream (null)
}
}
public static void main(String[] args)
throws IOException, InterruptedException {
SocketReader reader = new SocketReader("somehost", 25);
Thread thread = new Thread(reader);
thread.start();
Thread.sleep(1000);
thread.interrupt(); // Interrupt the thread
}
}
{code}
h2. Compliant Solution (Close Socket Connection)
This compliant solution terminates the blocking network I/O by closing the socket in the {{shutdown()}} method. The {{readLine()}} method throws a {{SocketException}} when the socket is closed, consequently allowing the thread to proceed. Note that it is impossible to keep the connection alive while simultaneously halting the thread both cleanly and immediately.
{code:bgColor=#ccccff}
public final class SocketReader implements Runnable {
// other methods...
public void readData() throws IOException {
String string;
try {
while ((string = in.readLine()) != null) {
// Blocks until end of stream (null)
}
} finally {
shutdown();
}
}
public void shutdown() throws IOException {
socket.close();
}
public static void main(String[] args)
throws IOException, InterruptedException {
SocketReader reader = new SocketReader("somehost", 25);
Thread thread = new Thread(reader);
thread.start();
Thread.sleep(1000);
reader.shutdown();
}
}
{code}
After the {{shutdown()}} method is called from {{main()}}, the {{finally}} block in {{readData()}} executes and calls {{shutdown()}} again, closing the socket for a second time. However, when the socket has already been closed, this second call does nothing.
When performing asynchronous I/O, a {{java.nio.channels.Selector}} can be unblocked by invoking either its {{close()}} or its {{wakeup()}} method.
When additional operations must be performed after emerging from the blocked state, use a {{boolean}} flag to indicate pending termination. When supplementing the code with such a flag, the {{shutdown()}} method should also set the flag to {{false}} so that the thread can cleanly exit from the while loop.
h2. Compliant Solution (Interruptible Channel)
This compliant solution uses an interruptible channel, {{java.nio.channels.SocketChannel}}, instead of a {{Socket}} connection. If the thread performing the network I/O is interrupted using the {{Thread.interrupt()}} method while it is reading the data, the thread receives a {{ClosedByInterruptException}}, and the channel is closed immediately. The thread's interrupted status is also set.
{code:bgColor=#ccccff}
public final class SocketReader implements Runnable {
private final SocketChannel sc;
private final Object lock = new Object();
public SocketReader(String host, int port) throws IOException {
sc = SocketChannel.open(new InetSocketAddress(host, port));
}
@Override public void run() {
ByteBuffer buf = ByteBuffer.allocate(1024);
try {
synchronized (lock) {
while (!Thread.interrupted()) {
sc.read(buf);
// ...
}
}
} catch (IOException ie) {
// Forward to handler
}
}
public static void main(String[] args)
throws IOException, InterruptedException {
SocketReader reader = new SocketReader("somehost", 25);
Thread thread = new Thread(reader);
thread.start();
Thread.sleep(1000);
thread.interrupt();
}
}
{code}
This technique interrupts the current thread. However, it stops the thread only because the code polls the thread's interrupted status with the {{Thread.interrupted()}} method and terminates the thread when it is interrupted. Using a {{SocketChannel}} ensures that the condition in the while loop is tested as soon as an interruption is received, even though the read is normally a blocking operation. Similarly, invoking the {{interrupt()}} method of a thread blocked on a {{java.nio.channels.Selector}} also causes that thread to awaken.
h2. Noncompliant Code Example (Database Connection)
This noncompliant code example shows a thread-safe {{DBConnector}} class that creates one JDBC connection per thread. Each connection belongs to one thread and is not shared by other threads. This is a common use case because JDBC connections are intended to be single-threaded.
{code:bgColor=#FFcccc}
public final class DBConnector implements Runnable {
private final String query;
DBConnector(String query) {
this.query = query;
}
@Override public void run() {
Connection connection;
try {
// Username and password are hard coded for brevity
connection = DriverManager.getConnection(
"jdbc:driver:name",
"username",
"password"
);
Statement stmt = connection.createStatement();
ResultSet rs = stmt.executeQuery(query);
// ...
} catch (SQLException e) {
// Forward to handler
}
// ...
}
public static void main(String[] args) throws InterruptedException {
DBConnector connector = new DBConnector("suitable query");
Thread thread = new Thread(connector);
thread.start();
Thread.sleep(5000);
thread.interrupt();
}
}
{code}
Database connections, like sockets, lack inherent interruptibility. Consequently, this design fails to support the client's attempts to cancel a task by closing the resource when the corresponding thread is blocked on a long-running query, such as a join.
h2. Compliant Solution ({{Statement.cancel()}})
This compliant solution uses a {{ThreadLocal}} wrapper around the connection so that a thread calling the {{initialValue()}} method obtains a unique connection instance. This approach allows provision of a {{cancelStatement()}} so that other threads or clients can interrupt a long-running query when required. The {{cancelStatement()}} method invokes the {{Statement.cancel()}} method.
{code:bgColor=#ccccff}
public final class DBConnector implements Runnable {
private final String query;
private volatile Statement stmt;
DBConnector(String query) {
this.query = query;
if (getConnection() != null) {
try {
stmt = getConnection().createStatement();
} catch (SQLException e) {
// Forward to handler
}
}
}
private static final ThreadLocal<Connection> connectionHolder =
new ThreadLocal<Connection>() {
Connection connection = null;
@Override public Connection initialValue() {
try {
// ...
connection = DriverManager.getConnection(
"jdbc:driver:name",
"username",
"password"
);
} catch (SQLException e) {
// Forward to handler
}
return connection;
}
};
public Connection getConnection() {
return connectionHolder.get();
}
public boolean cancelStatement() { // Allows client to cancel statement
if (stmt != null) {
try {
stmt.cancel();
return true;
} catch (SQLException e) {
// Forward to handler
}
}
return false;
}
@Override public void run() {
try {
if (stmt == null || (stmt.getConnection() != getConnection())) {
throw new IllegalStateException();
}
ResultSet rs = stmt.executeQuery(query);
// ...
} catch (SQLException e) {
// Forward to handler
}
// ...
}
public static void main(String[] args) throws InterruptedException {
DBConnector connector = new DBConnector("suitable query");
Thread thread = new Thread(connector);
thread.start();
Thread.sleep(5000);
connector.cancelStatement();
}
}
{code}
The {{Statement.cancel()}} method cancels the query, provided the database management system (DBMS) and driver both support cancellation. It is impossible to cancel the query if either the DBMS or the driver fail to support cancellation.
According to the Java API, interface {{Statement}} documentation \[[API 2006|AA. Bibliography#API 06]\]
{quote}
By default, only one {{ResultSet}} object per {{Statement}} object can be open at the same time. As a result, if the reading of one {{ResultSet}} object is interleaved with the reading of another, each must have been generated by different {{Statement}} objects.
{quote}
This compliant solution ensures that only one {{ResultSet}} is associated with the {{Statement}} belonging to an instance, and, consequently, only one thread can access the query results.
{mc} // Complete code for connecting to DB
String userName = "user";
String password = "pass";
String url = "jdbc:mysql://localhost:3306/dbname";
Class.forName ("com.mysql.jdbc.Driver").newInstance ();
connection = DriverManager.getConnection (url, userName, password);
{mc}
h2. Risk Assessment
Failure to provide facilities for thread termination can cause nonresponsiveness and DoS.
|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| THI04-J | low | probable | medium | {color:green}{*}P4{*}{color} | {color:green}{*}L3{*}{color} |
h2. Bibliography
| \[[API 2006|AA. Bibliography#API 06]\] | Class {{Thread}}, method {{stop}}, interface {{ExecutorService}} |
| \[[Darwin 2004|AA. Bibliography#Darwin 04]\] | 24.3, Stopping a Thread |
| \[[JDK7 2008|AA. Bibliography#JDK7 08]\] | Java Thread Primitive Deprecation |
| \[[JPL 2006|AA. Bibliography#JPL 06]\] | 14.12.1, Don't stopStop; 23.3.3, Shutdown Strategies |
| \[[JavaThreads 2004|AA. Bibliography#JavaThreads 04]\] | 2.4, Two Approaches to Stopping a Thread |
| \[[Goetz 2006|AA. Bibliography#Goetz 06]\] | Chapter 7, Cancellation and Shutdown |
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[!The CERT Oracle Secure Coding Standard for Java^button_arrow_left.png!|THI03-J. Always invoke wait() and await() methods inside a loop] [!The CERT Oracle Secure Coding Standard for Java^button_arrow_up.png!|09. Thread APIs (THI)] [!The CERT Oracle Secure Coding Standard for Java^button_arrow_right.png!|THI05-J. Do not use Thread.stop() to terminate threads]
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