...
Threads
...
and
...
tasks
...
that
...
block
...
on
...
operations
...
involving
...
network
...
or
...
file
...
input
...
/output
...
(IO)
...
must
...
provide
...
callers
...
with
...
an
...
explicit
...
termination
...
mechanism
...
to
...
prevent
...
denial
...
-of
...
-service
...
vulnerabilities.
...
Noncompliant
...
Code
...
Example
...
(blocking
...
IO,
...
volatile
...
flag)
...
This
...
noncompliant
...
code
...
example
...
uses
...
a
...
volatile
...
done
...
flag
...
to
...
indicate
...
that
...
it
...
is
...
safe
...
to
...
shut
...
down
...
the
...
thread,
...
as
...
suggested
...
in
...
guideline THI05-J.
...
...
...
...
...
...
...
...
.
...
However,
...
setting
...
the
...
flag
...
does
...
not
...
terminate
...
the
...
thread
...
if
...
it
...
is
...
blocked
...
on
...
network I/O as a consequence of invoking the readLine()
...
method.
| Code Block | ||||
|---|---|---|---|---|
| =
| |||
} public final class SocketReader implements Runnable { // Thread-safe class 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 IO, interruptible) This noncompliant code example is similar to the preceding one, but uses thread interruption to shut down the thread. Network IO is not responsive to thread interruption when a {{ |
Noncompliant Code Example (Blocking I/O, Interruptible)
This noncompliant code example is similar to the preceding one but uses thread interruption to shut down the thread. Network I/O is not responsive to thread interruption when a java.net.Socket
...
is
...
being
...
used.
...
The
...
readData()
...
and
...
main()
...
methods
...
are
...
modified
...
as
...
follows:
| Code Block | ||||
|---|---|---|---|---|
| =
| |||
} public final class SocketReader implements Runnable { // Thread-safe class // ... 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 |
Compliant Solution (close
...
socket
...
connection)
...
This
...
compliant
...
solution
...
resumes
...
the
...
thread
...
by
...
having
...
the
...
shutdown()
...
method
...
close
...
the
...
socket.
...
The
...
readLine()
...
method
...
throws
...
a
...
SocketException
...
when
...
the
...
socket
...
is
...
closed
...
which
...
lets
...
the
...
thread
...
proceed.
...
Note
...
that
...
there
...
is
...
no
...
way
...
to
...
keep
...
the
...
connection
...
alive
...
if
...
the
...
thread
...
is
...
to
...
be
...
cleanly
...
halted
...
immediately.
| Code Block | ||||
|---|---|---|---|---|
| =
| |||
} public final class SocketReader implements Runnable { // ... 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 {{ |
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,
...
this
...
second
...
call
...
has
...
no
...
effect
...
if
...
the
...
socket
...
has
...
already
...
been
...
closed.
...
When
...
performing
...
asynchronous
...
I/O,
...
a
...
java.nio.channels.Selector
...
may
...
also
...
be
...
brought
...
out
...
of
...
the
...
blocked
...
state
...
by
...
either
...
invoking
...
its
...
close()
...
or
...
wakeup()
...
method.
...
A
...
boolean
...
flag
...
can
...
be
...
used
...
if
...
additional
...
operations
...
need
...
to
...
be
...
performed
...
after
...
emerging
...
from
...
the
...
blocked
...
state.
...
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.
...
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 Block | ||||
|---|---|---|---|---|
| =
| |||
} 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
...
only
...
stops
...
the
...
thread
...
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, despite the read being a blocking operation. Similarly, invoking the interrupt() method of a thread that is blocked because of java.nio.channels.Selector
...
also
...
causes
...
that
...
thread
...
to
...
awaken.
...
Noncompliant Code Example (database
...
connection)
...
This
...
noncompliant
...
code
...
example
...
shows
...
a
...
thread-safe DBConnector class that creates one Java Database Connectivity (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 not meant to be shared by multiple threads.
| Code Block | ||
|---|---|---|
| ||
class {{DBConnector}} 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 not meant to be shared by multiple threads. {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,
...
are
...
not
...
inherently
...
interruptible.
...
Consequently,
...
this
...
design
...
does
...
not
...
permit
...
a
...
client
...
to
...
cancel
...
a
...
task
...
by
...
closing
...
the
...
resource
...
if
...
the
...
corresponding
...
thread
...
is
...
blocked
...
on
...
a
...
long
...
-running
...
query
...
such
...
as
...
a
...
join.
...
Compliant Solution (Statement.cancel()
...
)
...
This
...
compliant
...
solution
...
uses
...
a
...
ThreadLocal
...
wrapper
...
around
...
the
...
connection
...
so
...
that
...
a
...
thread that calling the initialValue() method obtains a unique connection instance. The advantage of this approach is that a cancelStatement() method can be provided so that other threads or clients can interrupt a long-running query when required. The cancelStatement() method invokes the Statement.cancel() method.
| Code Block | ||
|---|---|---|
| ||
that calls {{initialValue()}} obtains a unique connection instance. The advantage of this approach is that a {{cancelStatement()}} method can be provided so that other threads or clients can interrupt a long running query when required. The {{cancelStatement()}} method invokes {{Statement.cancel()}}. {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 {{ |
The Statement.cancel()
...
method
...
cancels
...
the
...
query
...
provided
...
that
...
the
...
database management system (DBMS) and driver both support cancellation. It is not possible to conform with this guideline if they do not.
| Wiki Markup |
|---|
According to the Java API, interface {{Statement}} documentation |
...
\[[API |
...
2006|AA. Java References#API 06]\] |
...
By default,
...
only
...
one
...
ResultSet
...
object
...
per
...
Statement
...
object
...
can
...
be
...
open
...
at
...
the
...
same
...
time.
...
Therefore,
...
if
...
the
...
reading
...
of
...
one
...
ResultSetobject is interleaved with the reading of another, each must have been generated by differentStatementobjects.
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.
Risk Assessment
Failing to provide facilities for thread termination can cause non-responsiveness and denial of service.
Guideline | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
THI06- J | low | probable | medium | P4 | L3 |
References
| Wiki Markup |
|---|
\[[API 2006|AA. Java References#API 06]\] Class Thread, method {{stop}}, interface ExecutorService \[[Darwin 2004|AA. Java References#Darwin 04]\] 24.3 Stopping a Thread \[[ |
...
JDK7 |
...
2008|AA. Java |
...
References#JDK7 |
...
08]\] |
...
Concurrency Utilities, More information: Java Thread Primitive Deprecation
\[[JPL 2006|AA. Java References#JPL 06]\] 14.12.1. Don't stop and 23.3.3. Shutdown Strategies
\[[JavaThreads 2004|AA. Java References#JavaThreads 04]\] 2.4 Two Approaches to Stopping a Thread
\[[Goetz 2006|AA. Java References#Goetz 06]\] Chapter 7: Cancellation and shutdown |
...
LCK09-J. Do not perform operations that may block while holding a lock 12. Locking (LCK) LCK10-J. Do not use incorrect forms of the double-checked locking idiom