...
- 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 recover from the exceptional condition effectively. Also, there is nothing stopping a thread from throwing anotherThreadDeathexception while recovery is in progress.
Noncompliant Code Example
This noncompliant code example shows a thread that forcefully comes to a halt when the Thread.stop() method is invoked. Neither the catch nor the finally block is executed. Needless to say, any monitors that are held are immediately released, leaving the object in a delicate state.
| Code Block | ||
|---|---|---|
| ||
class BadStop implements Runnable {
public void run() {
try {
Thread.currentThread().sleep(1000);
} catch(InterruptedException ie) { // Not executed
System.out.println("Performing cleanup");
} finally { // Not executed
System.out.println("Closing resources");
}
System.out.println("Done!");
}
}
class Controller {
public static void main(String[] args) {
Thread t = new Thread(new BadStop());
t.start();
t.interrupt(); // Artificially induce an InterruptedException
t.stop(); // Force thread cancellation
}
}
|
Compliant Solution (1)
This compliant example uses a boolean flag called done to indicate whether the thread should be stopped after any necessary cleanup code has finished executing. An accessor method shutdown() is used to set the flag to true, after which the thread can start the cancellation process. The done flag is also set immediately after the execution of the finally block's resource clean-up statements so that the system does not continue relinquishing resources that it has already released, in the event of the done flag staying false.
| Code Block | ||
|---|---|---|
| ||
class ControlledStop implements Runnable{
protected volatile boolean done = false;
public void run() {
while(!done) {
try {
Thread.currentThread().sleep(1000);
} catch(InterruptedException ie) {
System.out.println("Interrupted Exception");
// Handle the exception
} finally {
System.out.println("Closing resources");
done = true;
}
}
System.out.println("Done!");
}
protected void shutdown(){
done = true;
}
}
class Controller {
public static void main(String[] args) throws InterruptedException {
ControlledStop c = new ControlledStop();
Thread t = new Thread(c);
t.start();
t.interrupt(); // Artificially induce an InterruptedException
Thread.sleep(1000); // Wait for some time to allow the exception
// to be caught (demonstration only)
c.shutdown();
}
}
|
Compliant Solution (2)
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
This noncompliant code example uses the advice suggested in the previous compliant solution. Unfortunately, this does not help in terminating the thread because it is blocked on some network IO because of the readLine() method. The boolean flag trick does not work in such cases; a good alternative to end the thread is required.
| Code Block | ||
|---|---|---|
| ||
class StopSocket extends Thread {
protected Socket s;
protected volatile boolean done = false;
public void run() {
while(!done) {
try {
s = new Socket("somehost", 25);
BufferedReader br = new BufferedReader(new InputStreamReader(s.getInputStream()));
String s = null;
while((s = br.readLine()) != null) {
// Blocks until end of stream (null)
}
System.out.println("Blocked, will not get executed until some data is received. " + s);
} catch (IOException ie) {
System.out.println("IO Exception");
// Handle the exception
} finally {
System.out.println("Closing resources");
done = true;
}
}
}
public void shutdown() throws IOException {
done = true;
}
}
class Controller {
public static void main(String[] args) throws InterruptedException, IOException {
StopSocket ss = new StopSocket();
Thread t = new Thread(ss);
t.start();
Thread.sleep(1000);
ss.shutdown();
}
}
|
Compliant Solution
This compliant solution simply closes the socket connection, both using the shutdown() method as well as the finally block. As a result, the thread is bound to stop due to a SocketException. Note that there is no way to keep the connection alive if the thread is to be cleanly halted immediately.
| Code Block | ||
|---|---|---|
| ||
class StopSocket extends Thread {
protected Socket s;
public void run() {
try {
s = new Socket("somehost", 25);
BufferedReader br = new BufferedReader(new InputStreamReader(s.getInputStream()));
String s = null;
while((s = br.readLine()) != null) {
// Blocks until end of stream (null)
}
System.out.println("Blocked, will not get executed until some data is received. " + s);
} catch (IOException ie) {
System.out.println("IO Exception");
// Handle the exception
} finally {
System.out.println("Closing resources");
try {
if(s != null)
s.close();
} catch (IOException e) { /* Forward to handler */ }
}
}
public void shutdown() throws IOException {
if(s != null)
s.close();
}
}
class Controller {
public static void main(String[] args) throws InterruptedException, IOException {
StopSocket ss = new StopSocket();
Thread t = new Thread(ss);
t.start();
Thread.sleep(1000);
ss.shutdown();
}
}
|
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 |
|---|---|---|---|---|---|
CON35- 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
| Wiki Markup |
|---|
\[[API 06|AA. Java References#API 06]\] Class Thread, method {{stop}}
\[[Darwin 04|AA. Java References#Darwin 04]\] 24.3 Stopping a Thread
\[[JDK7 08|AA. Java References#JDK7 08]\] Concurrency Utilities, More information: Java Thread Primitive Deprecation
\[[JPL 05|AA. Java References#JPL 05]\] 14.12.1. Don't stop
\[[JavaThreads 04|AA. Java References#JavaThreads 04]\] 2.4 Two Approaches to Stopping a Thread |
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