java data matrix reader Coping with hung threads in Java

Printing Data Matrix in Java Coping with hung threads

4.1.3 Coping with hung threads
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Time and time again, frozen threads wreak the most havoc and pose the greatest threat to an application s stability. As with almost all thread-related code, tracking down bugs can be a daunting task. Java threads hang for a number of reasons. First, you have deadlocks. For example, when thread A holds a lock to resource A and waits for resource B, and thread B holds onto resource B and waits for resource A, both threads will wait in an infinite, deadly embrace. As another example, consider an object pool with four objects, where threads A and B each need three objects to complete their task. If thread A holds two objects and thread B holds two objects, the threads will deadlock waiting for a third. When resource pools have set size limits such as this, they should take care to implement timeouts for waiting clients to prevent deadlocks. Most application server database connection pools do exactly this. A second and more complicated situation occurs during blocked I/O for example, while creating a connection, opening a stream, or reading from an input stream. Solving these situations is a little less straightforward as some I/O framework implementations ignore timeouts. These situations can take down an application server easily as clients place multiple requests, eventually hanging
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every thread in the server s thread pool until the application can service no more requests. In either case, your best comrade in tracking down the cause or causes of a deadlock is the thread dump. You can trigger a thread dump on a running application by hitting Ctrl+Break in a Windows environment or by invoking a kill -3 against the process in a Unix environment. In both situations, the virtual machine prints a thread dump to standard out (listing 4.1):
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Listing 4.1 A sample thread dump
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"Thread-2" prio=5 tid=0x119570 nid=0x11 waiting for monitor entry [f1681000..f168199c] at MyClass.run(Hang.java:54) - waiting to lock (a java.lang.Object) - locked (a java.lang.String) "Thread-3" prio=5 tid=0x118a18 nid=0x10 waiting for monitor entry [f1781000..f178199c] at MyClass.run(Hang.java:54) - waiting to lock (a java.lang.String) - locked (a java.lang.Object) "Thread-1" prio=5 tid=0x117660 nid=0xf waiting on condition [f1881000..f188199c] at java.lang.Thread.sleep(Native Method) at Deadlock$3.run(Hang.java:35) - locked (a java.lang.Object) at java.lang.Thread.run(Thread.java:536)
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As you can see in listing 4.1, a thread dump looks a lot like an exception stack trace. The dump shows the call stack for every thread in the VM in addition to what locks the thread holds, where it s holding them, what locks the thread waits for and where it waits for them. Many times, when a thread hangs due to blocked I/O, you can do little to recover. You can, however, do your best to isolate temperamental operations from other operations and the client. We once worked on an application that accessed a highly secure database. The database required user-based authentication for each request, coupled with other implementation limitations, which forced a new connection for almost every request. (Users ran one or two queries at a time, negating any benefits that might be derived from pooling.) One day we came in to find that the entire application had inexplicably crashed, with no sign of a pending return, and our team was listed at the top of
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the company vice president s daily report (not a good place to be questions are asked, bonuses lost; you get the picture). When the database in question went down, threads attempting to open the database connection simply hung. Even worse, thanks to thread dumps, we discovered that the crashing code used java.sql.DriverManager.getConnection(String url) to open connections. The DriverManager implementation synchronizes all access to the class. Once the first thread hung waiting for a connection, any other thread accessing the DriverManager class hung as well. This occurred whether or not a second thread accessed the inoperable database, because the original thread still held a lock on the class s synchronization monitor. As the clients tried apparently failed requests again, more and more threads locked up, until no more threads existed to service client requests (figure 4.1).
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