...
Each
...
thread
...
in
...
Java
...
is
...
assigned
...
to
...
a
...
thread
...
group
...
upon
...
the
...
thread's
...
creation.
...
These
...
groups
...
are
...
implemented
...
by
...
the
...
java.lang.ThreadGroup
...
class.
...
When
...
the
...
thread
...
group
...
name
...
is
...
not
...
specified
...
explicitly,
...
the
...
main
...
default
...
group
...
is
...
assigned
...
by
...
the
...
Java
...
Virtual
...
Machine
...
(JVM)
...
[
...
...
...
].
...
The
...
convenience
...
methods
...
of
...
the
...
ThreadGroup
...
class
...
can
...
be
...
used
...
to
...
operate
...
on
...
all
...
threads
...
belonging
...
to
...
a
...
thread
...
group
...
at
...
once.
...
For
...
instance,
...
the
...
ThreadGroup.interrupt()
...
method
...
interrupts
...
all
...
threads
...
in
...
the
...
thread
...
group.
...
Thread
...
groups
...
also
...
help
...
reinforce
...
layered
...
security
...
by
...
confining
...
threads
...
into
...
groups
...
so
...
that
...
they
...
avoid
...
interference
...
with
...
threads
...
in
...
other
...
groups
...
[
...
...
...
].
...
Even
...
though
...
thread
...
groups
...
are
...
useful
...
for
...
keeping
...
threads
...
organized,
...
programmers
...
seldom
...
benefit
...
from
...
their
...
use
...
because
...
many
...
of
...
the
...
methods
...
of
...
the
...
ThreadGroup
...
class
...
are
...
deprecated
...
(for
...
example,
...
allowThreadSuspension(),
...
resume(),
...
stop(),
...
and
...
suspend()
...
).
...
Furthermore,
...
many
...
nondeprecated
...
methods
...
are
...
obsolete
...
in
...
that
...
they
...
offer
...
little
...
desirable
...
functionality.
...
Ironically,
...
a
...
few
...
ThreadGroup
...
methods
...
are
...
not
...
even
...
thread-safe
...
...
...
]
...
.
...
Insecure
...
yet
...
nondeprecated
...
methods
...
include
ThreadGroup.activeCount()
...
According
...
- to
...
- the
...
- Java
...
- API,
...
- the
...
activeCount()
...
- method
...
- [API
...
- 2006]
This method is often used as a precursor to thread enumeration. Threads that have never started nevertheless reside in the thread group and are considered to be active. The active count is also affected by the presence of certain system threads [API 2006]. Consequently, thereturns an estimate of the number of active threads in this thread group.
activeCount()method might fail to reflect the actual number of running tasks in the thread group.
ThreadGroup.enumerate()
According to the Java API,ThreadGroupclass documentation [API 2006][The
enumerate()method] copies into the specified array every active thread in this thread group and its subgroups. An application should use theactiveCountmethod to get an estimate of how big the array should be. If the array is too short to hold all the threads, the extra threads are silently ignored.
Using the ThreadGroup APIs to shut down threads also has pitfalls. Because the stop() method is deprecated, programs require alternative methods to stop threads. According to The Java Programming Language [JPL 2006]
One way is for the thread initiating the termination to join the other threads and so know when those threads have terminated. However, an application may have to maintain its own list of the threads it creates because simply inspecting the
ThreadGroupmay return library threads that do not terminate and for which join will not return.
The Executor framework provides a better API for managing a logical grouping of threads and offers secure facilities for handling shutdown and thread exceptions [Bloch 2008]. Consequently, programs must not invoke ThreadGroup methods.
Noncompliant Code Example
This noncompliant code example contains a NetworkHandler class that maintains a controller thread. The controller thread delegates each new request to a worker thread. To demonstrate the race condition in this example, the controller thread serves three requests by starting three threads in succession from its run() method. All threads are defined to belong to the Chief thread group.
| Code Block | ||
|---|---|---|
| ||
|AA. References#API 06]\] {quote} returns an estimate of the number of active threads in this thread group. {quote} This method is often used as a precursor to thread enumeration. Threads that have never started nevertheless reside in the thread group and are considered to be active. The active count is also affected by the presence of certain system threads \[[API 2006|AA. References#API 06]\]. Consequently, the _{{activeCount()}}_ method might fail to reflect the actual number of running tasks in the thread group. * {{ThreadGroup.enumerate()}} According to the Java API, {{ThreadGroup}} class documentation \[[API 2006|AA. References#API 06]\] {quote} \[The {{enumerate()}} method\] copies into the specified array every active thread in this thread group and its subgroups. An application should use the {{activeCount}} method to get an estimate of how big the array should be. If the array is too short to hold all the threads, the extra threads are silently ignored. {quote} Using the {{ThreadGroup}} APIs to shut down threads also has pitfalls. Because the {{stop()}} method is deprecated, programs require alternative methods to stop threads. According to _The Java Programming Language_ \[[JPL 2006|AA. References#JPL 06]\] {quote} One way is for the thread initiating the termination to join the other threads and so know when those threads have terminated. However, an application may have to maintain its own list of the threads it creates because simply inspecting the {{ThreadGroup}} may return library threads that do not terminate and for which join will not return. {quote} The {{Executor}} framework provides a better API for managing a logical grouping of threads and offers secure facilities for handling shutdown and thread exceptions \[[Bloch 2008|AA. References#Bloch 08]\]. Consequently, programs must not invoke {{ThreadGroup}} methods. h2. Noncompliant Code Example This noncompliant code example contains a {{NetworkHandler}} class that maintains a {{controller}} thread. The {{controller}} thread delegates each new request to a worker thread. To demonstrate the race condition in this example, the {{controller}} thread serves three requests by starting three threads in succession from its {{run()}} method. All threads are defined to belong to the {{Chief}} thread group. {code:bgColor=#FFcccc} final class HandleRequest implements Runnable { public void run() { // Do something } } public final class NetworkHandler implements Runnable { private static ThreadGroup tg = new ThreadGroup("Chief"); @Override public void run() { new Thread(tg, new HandleRequest(), "thread1").start(); new Thread(tg, new HandleRequest(), "thread2").start(); new Thread(tg, new HandleRequest(), "thread3").start(); } public static void printActiveCount(int point) { System.out.println("Active Threads in Thread Group " + tg.getName() + " at point(" + point + "):" + " " + tg.activeCount()); } public static void printEnumeratedThreads(Thread[] ta, int len) { System.out.println("Enumerating all threads..."); for (int i = 0; i < len; i++) { System.out.println("Thread " + i + " = " + ta[i].getName()); } } public static void main(String[] args) throws InterruptedException { // Start thread controller Thread thread = new Thread(tg, new NetworkHandler(), "controller"); thread.start(); // Gets the active count (insecure) Thread[] ta = new Thread[tg.activeCount()]; printActiveCount(1); // P1 // Delay to demonstrate TOCTOU condition (race window) Thread.sleep(1000); // P2: the thread count changes as new threads are initiated printActiveCount(2); // Incorrectly uses the (now stale) thread count obtained at P1 int n = tg.enumerate(ta); // Silently ignores newly initiated threads printEnumeratedThreads(ta, n); // (between P1 and P2) // This code destroys the thread group if it does // not have any live threads for (Thread thr : ta) { thr.interrupt(); while(thr.isAlive()); } tg.destroy(); } } {code} |
This
...
implementation
...
contains
...
a
...
time-of-check,
...
time-of-use
...
(TOCTOU)
...
vulnerability,
...
because
...
it
...
obtains
...
the
...
count
...
and
...
enumerates
...
the
...
list
...
without
...
ensuring
...
atomicity.
...
If
...
one
...
or
...
more
...
new
...
requests
...
were
...
to
...
occur
...
after
...
the
...
call
...
to
...
activeCount()
...
and
...
before
...
the
...
call
...
to
...
enumerate()
...
in
...
the
...
main()
...
method,
...
the
...
total
...
number
...
of
...
threads
...
in
...
the
...
group
...
would
...
increase,
...
but
...
the
...
enumerated
...
list
...
ta
...
would
...
contain
...
only
...
the
...
initial
...
number,
...
that
...
is,
...
two
...
thread
...
references:
...
main
...
and
...
controller
...
.
...
Consequently,
...
the
...
program
...
would
...
fail
...
to
...
account
...
for
...
the
...
newly
...
started
...
threads
...
in
...
the
...
Chief
...
thread
...
group.
...
Any
...
subsequent
...
use
...
of
...
the
...
ta
...
array
...
would
...
be
...
insecure.
...
For
...
example,
...
calling
...
the
...
destroy()
...
method
...
to
...
destroy
...
the
...
thread
...
group
...
and
...
its
...
sub-groups
...
would
...
not
...
work
...
as
...
expected.
...
The
...
precondition
...
to
...
calling
...
destroy()
...
is
...
that
...
the
...
thread
...
group
...
must
...
be
...
empty
...
with
...
no
...
executing
...
threads.
...
The
...
code
...
attempts
...
to
...
comply
...
with
...
the
...
precondition
...
by
...
interrupting
...
every
...
thread
...
in
...
the
...
thread
...
group.
...
However,
...
the
...
thread
...
group
...
would
...
not
...
be
...
empty
...
when
...
the
...
destroy()
...
method
...
was
...
called,
...
causing
...
a
...
java.lang.IllegalThreadStateException
...
to
...
be
...
thrown.
Compliant Solution
This compliant solution uses a fixed thread pool rather than a ThreadGroup to group its three tasks. The java.util.concurrent.ExecutorService
...
interface
...
provides
...
methods
...
to
...
manage
...
the
...
thread
...
pool.
...
Although
...
the
...
interface
...
lacks
...
methods
...
for
...
finding
...
the
...
number
...
of
...
actively
...
executing
...
threads
...
or
...
for
...
enumerating
...
the
...
threads,
...
the
...
logical
...
grouping
...
can
...
help
...
control
...
the
...
behavior
...
of
...
the
...
group
...
as
...
a
...
whole.
...
For
...
instance,
...
invoking
...
the
...
shutdownPool()
...
method
...
terminates
...
all
...
threads
...
belonging
...
to
...
a
...
particular
...
thread
...
pool.
| Code Block | ||||
|---|---|---|---|---|
| =
| |||
} public final class NetworkHandler { private final ExecutorService executor; NetworkHandler(int poolSize) { this.executor = Executors.newFixedThreadPool(poolSize); } public void startThreads() { for (int i = 0; i < 3; i++) { executor.execute(new HandleRequest()); } } public void shutdownPool() { executor.shutdown(); } public static void main(String[] args) { NetworkHandler nh = new NetworkHandler(3); nh.startThreads(); nh.shutdownPool(); } } {code} |
Before
...
Java
...
SE
...
5.0,
...
applications
...
that
...
needed
...
to
...
catch
...
an
...
uncaught
...
exception
...
in
...
a
...
separate
...
thread
...
had
...
to
...
extend
...
the
...
ThreadGroup
...
class
...
because
...
this
...
was
...
the
...
only
...
direct
...
approach
...
to
...
provide
...
the
...
required
...
functionality.
...
Specifically,
...
an
...
application's
...
UncaughtExceptionHandler
...
could
...
only
...
be
...
controlled
...
by
...
subclassing
...
ThreadGroup
...
.
...
In
...
more
...
recent
...
versions
...
of
...
Java,
...
UncaughtExceptionHandler
...
is
...
maintained
...
on
...
a
...
per-thread
...
basis
...
using
...
an
...
interface
...
enclosed
...
by
...
the
...
Thread
...
class.
...
Consequently,
...
the
...
ThreadGroup
...
class
...
provides
...
little
...
unique
...
functionality
...
[
...
...
...
],
...
...
...
].
...
Refer
...
to
...
rule
...
...
...
...
...
...
...
...
...
...
...
...
...
...
for
...
more
...
information
...
on
...
using
...
uncaught
...
exception
...
handlers
...
in
...
thread
...
pools.
...
Risk Assessment
Use of the ThreadGroup APIs may result in race conditions, memory leaks, and inconsistent object state.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
THI01-J | low | probable | medium | P4 | L3 |
Bibliography
[API 2006] | Methods |
Item 53. Avoid thread groups | |
Item 73. Avoid thread groups | |
Section 7.3.1, |
...
Uncaught |
...
Exception |
...
Handlers | |
13.1, |
...
|
[JPL |
...
...
] | 23.3.3, |
...
Shutdown |
...
Strategies | |
[SDN 2006] | Bug ID 4089701 and 4229558 |
|
...
09. Thread APIs (THI) THI02-J. Notify all waiting threads rather than a single thread