Threads always preserve class invariants when they are allowed to exit normally. Programmers often try to forcefully terminate threads when they believe that the task is accomplished, the request has been canceled or the program needs to quickly shutdown.
A few thread APIs were introduced to facilitate thread suspension, resumption and termination but were later deprecated because of inherent design weaknesses. The Thread.stop() method is one example. It throws a ThreadDeath exception to stop the thread. Two cases arise:
- If
ThreadDeathis left uncaught, it allows the execution of afinallyblock which performs the usual cleanup operations. Use of theThread.stop()method is highly inadvisable because of two reasons. First, no particular thread can be forcefully stopped because an arbitrary thread can catch the thrownThreadDeathexception and simply choose to ignore it. Second, abruptly stopping a thread results in the release of all the locks that it has acquired, violating the guarantees provided by the critical sections. Moreover, the objects end up in an inconsistent state, non-deterministic behavior being a typical outcome.
- As a remediation measure, catching the
ThreadDeathexception on the other hand can itself ensnarl multithreaded code. For one, the exception can be thrown anywhere, making it difficult to trace and effectively recover from the exceptional condition. Also, there is nothing stopping a thread from throwing anotherThreadDeathexception while recovery is in progress.
More information about deprecated methods is available in MET15-J. Do not use deprecated or obsolete methods.
Noncompliant Code Example (Deprecated Thread.stop())
This noncompliant code example shows a thread that fills a vector with pseudorandom numbers. The thread is stopped after a fixed amount of time.
public class Container implements Runnable {
private final Vector<Integer> vector = new Vector<Integer>();
public Vector<Integer> getVector() {
return vector;
}
public synchronized void run() {
Random number = new Random(123L);
int i = 10;
while (i > 0) {
vector.add(number.nextInt(100));
i--;
}
}
public static void main(String[] args) throws InterruptedException {
Thread thread = new Thread(new Container());
thread.start();
Thread.sleep(5000);
thread.stop();
}
}
Because the class Vector is thread-safe, operations performed by multiple threads on its shared instance are expected to leave it in a consistent state. For instance, the Vector.size() method always reflects the true number of elements in the vector even when an element is added or removed. This is because the vector instance uses its own intrinsic lock to prevent other threads from accessing it while its state is temporarily inconsistent.
However, the Thread.stop() method causes the thread to stop what it is doing and throw a ThreadDeath exception, and release all locks that it has acquired [[API 06]]. If the thread is in the process of adding a new integer to the vector when it is stopped, the vector may become accessible while it is in an inconsistent state. For example, Vector.size() may be two while the vector contains three elements (the element count is incremented after adding the element).
Compliant Solution (volatile flag)
This compliant solution stops the thread by using a volatile flag. An accessor method shutdown() is used to set the flag to true. The thread's run() method polls the done flag, and shuts down when it becomes true.
public class Container implements Runnable {
private final Vector<Integer> vector = new Vector<Integer>();
private volatile boolean done = false;
public Vector<Integer> getVector() {
return vector;
}
public void shutdown() {
done = true;
}
public synchronized void run() {
Random number = new Random(123L);
int i = 10;
while (!done && i > 0) {
vector.add(number.nextInt(100));
i--;
}
}
public static void main(String[] args) throws InterruptedException {
Container container = new Container();
Thread thread = new Thread(container);
thread.start();
Thread.sleep(5000);
container.shutdown();
}
}
Compliant Solution (Interruptible)
This compliant solution stops the thread by using the Thread.interrupted() method.
public class Container implements Runnable {
private final Vector<Integer> vector = new Vector<Integer>();
public Vector<Integer> getVector() {
return vector;
}
public synchronized void run() {
Random number = new Random(123L);
int i = 10;
while (!Thread.interrupted() && i > 0) {
vector.add(number.nextInt(100));
i--;
}
}
public static void main(String[] args) throws InterruptedException {
Container c = new Container();
Thread thread = new Thread(c);
thread.start();
Thread.sleep(5000);
thread.interrupt();
}
}
This method interrupts the current thread, however, it only stops the thread because the thread logic polls the interrupted flag using the method Thread.interrupted(), and shuts down when it is interrupted.
Compliant Solution (RuntimePermission stopThread)
Remove the default permission java.lang.RuntimePermission stopThread from the security policy file to deny the Thread.stop() invoking code, the required privileges.
Noncompliant Code Example (blocking IO, volatile flag)
This noncompliant code example uses a volatile done flag to indicate when the thread should shut down, as suggested above. However, this does not help in terminating the thread because it is blocked on some network IO as a consequence of using the readLine() method.
class SocketReader implements Runnable {
private final Socket socket;
private final BufferedReader in;
private volatile boolean done = false;
private final Object lock = new Object();
private boolean isRunning = false;
public static class MultipleUseException extends RuntimeException {};
public SocketReader() throws IOException {
this.socket = new Socket("somehost", 25);
this.in = new BufferedReader(new InputStreamReader(this.socket.getInputStream()));
}
// Only one thread can use the socket at a particular time
public void run() {
try {
synchronized (lock) {
if (isRunning) {
throw new MultipleUseException();
}
isRunning = true;
}
execute();
} catch (IOException ie) {
// Forward to handler
}
}
public void execute() 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();
Thread thread = new Thread(reader);
thread.start();
Thread.sleep(1000);
reader.shutdown();
}
}
Note that the Runnable object prevents itself from being run in multiple threads using an isRunning() flag. If one thread tries to invoke run() after another thread has already done so, the method will exit with an exception. This exception is not recoverable, as it indicates a bug in the proper usage of this class, and hence, it may be unchecked.
Noncompliant Code Example (blocking IO, interruptible)
This noncompliant code example uses thread interruption to indicate when the thread should shut down, as suggested above. However, this does not help in terminating the thread because it is blocked on some network IO as a consequence of using the readLine() method. Network I/O is not responsive to thread interruption.
class SocketReader implements Runnable {
// ...
public void execute() 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();
Thread thread = new Thread(reader);
thread.start();
Thread.sleep(1000);
thread.interrupt();
}
}
Compliant Solution (close socket connection)
This compliant solution closes the socket connection, by having the shutdown() method close the socket. As a result, the thread is bound to stop because of a SocketException. Note that there is no way to keep the connection alive if the thread is to be cleanly halted immediately.
class SocketReader implements Runnable {
// ...
public void execute() 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();
Thread thread = new Thread(reader);
thread.start();
Thread.sleep(1000);
reader.shutdown();
}
}
A boolean flag can be used (as described earlier) if additional clean-up operations need to be performed.
Compliant Solution (interruptible channel)
This compliant solution uses an interruptible channel, SocketChannel instead of a Socket connection. If the thread performing the network IO is interrupted using the Thread.interrupt() method, for instance, while reading the data, the thread receives a ClosedByInterruptException and the channel is closed immediately. The thread's interrupt status is also set.
class SocketReader implements Runnable {
private final SocketChannel sc;
private final Object lock = new Object();
private boolean isRunning = false;
private class MultipleUseException extends RuntimeException {};
public SocketReader() throws IOException {
sc = SocketChannel.open(new InetSocketAddress("somehost", 25));
}
public void run() {
synchronized (lock) {
if (isRunning) {
throw new MultipleUseException();
}
isRunning = true;
}
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();
Thread thread = new Thread(reader);
thread.start();
Thread.sleep(1000);
thread.interrupt();
}
}
This method interrupts the current thread, however, it only stops the thread because the thread logic polls the interrupted flag using the method Thread.interrupted(), and shuts down when it is interrupted.
Risk Assessment
Trying to force thread shutdown can result in inconsistent object state and corrupt the object. Critical resources may also leak if cleanup operations are not carried out as required.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
CON13- J |
low |
probable |
medium |
P4 |
L3 |
Automated Detection
TODO
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
[[API 06]] Class Thread, method stop
[[Darwin 04]] 24.3 Stopping a Thread
[[JDK7 08]] Concurrency Utilities, More information: Java Thread Primitive Deprecation
[[JPL 06]] 14.12.1. Don't stop and 23.3.3. Shutdown Strategies
[[JavaThreads 04]] 2.4 Two Approaches to Stopping a Thread
CON12-J. Avoid deadlock by requesting and releasing locks in the same order 11. Concurrency (CON) VOID CON14-J. Ensure atomicity of 64-bit operations