How to Analyze Java Thread / Heap Dumps

The content of this article was originally written by Tae Jin Gu on the Cubrid blog.

When there is an obstacle, or when a Java based Web application is running much slower than expected, we need to use thread dumps. If thread dumps feel like very complicated to you, this article may help you very much. Here I will explain what threads are in Java, their types, how they are created, how to manage them, how you can dump threads from a running application, and finally how you can analyze them and determine the bottleneck or blocking threads. This article is a result of long experience in Java application debugging.

Java and Thread

A web server uses tens to hundreds of threads to process a large number of concurrent users. If two or more threads utilize the same resources, a contention between the threads is inevitable, and sometimes deadlock occurs.

Thread contention is a status in which one thread is waiting for a lock, held by another thread, to be lifted. Different threads frequently access shared resources on a web application. For example, to record a log, the thread trying to record the log must obtain a lock and access the shared resources.

Deadlock is a special type of thread contention, in which two or more threads are waiting for the other threads to complete their tasks in order to complete their own tasks.

Different issues can arise from thread contention. To analyze such issues, you need to use the thread dump. A thread dump will give you the information on the exact status of each thread.

Background Information for Java Threads

Thread Synchronization

A thread can be processed with other threads at the same time. In order to ensure compatibility when multiple threads are trying to use shared resources, one thread at a time should be allowed to access the shared resources by using thread synchronization.
Thread synchronization on Java can be done using monitor. Every Java object has a single monitor. The monitor can be owned by only one thread. For a thread to own a monitor that is owned by a different thread, it needs to wait in the wait queue until the other thread releases its monitor.

Thread Status

In order to analyze a thread dump, you need to know the status of threads. The statuses of threads are stated on java.lang.Thread.State.
 
Figure 1: Thread Status.

• NEW: The thread is created but has not been processed yet.
• RUNNABLE: The thread is occupying the CPU and processing a task. (It may be in WAITING status due to the OS's resource distribution.)
• BLOCKED: The thread is waiting for a different thread to release its lock in order to get the monitor lock.
• WAITING: The thread is waiting by using a wait, join or park method.
• TIMED_WAITING: The thread is waiting by using a sleep, wait, join or park method. (The difference from WAITING is that the maximum waiting time is specified by the method parameter, and WAITING can be relieved by time as well as external changes.) 

Thread Types

Java threads can be divided into two:

1. daemon threads;
2. and non-daemon threads.

Daemon threads stop working when there are no other non-daemon threads. Even if you do not create any threads, the Java application will create several threads by default. Most of them are daemon threads, mainly for processing tasks such as garbage collection or JMX.
A thread running the 'static void main(String[] args)’ method is created as a non-daemon thread, and when this thread stops working, all other daemon threads will stop as well. (The thread running this main method is called the VM thread in HotSpot VM.)

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Commands to create thread dump on solaris:

[user-account@localhost]$ ps -ef | grep java
To list all running java pid 
[user-account@localhost]$ pargs pid
To determine which pid is for dump 
[user-account@localhost]$ cd %JAVA_HOME%/bin
Go to java binary folder which having jstack command
[user-account@localhost]$ ./jstack -l pid /var/tmp/thread-dump-pid.out
Create the running thread dump

Solaris CMD QRC From Internet

My Google link

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