Welcome!

IBM Cloud Authors: Pat Romanski, Liz McMillan, Yeshim Deniz, Carmen Gonzalez, Elizabeth White

Related Topics: Java IoT, Weblogic

Java IoT: Article

Java Thread Dumps

Analyzing Java Thread Dumps

Software maintenance is a tedious and challenging job. As long as the software functions as expected, it looks great. Imagine the situation, your pager keeps buzzing in the midnight (Not a happy feeling, right?).

Any software system, no matter how well it has been built and quality tested might get into run-time performance issues. The reasons can be within the functional boundary or can be from the external environment. Software systems are built on certain assumptions and pre-conceived notions. However, when they go live, the assumptions may turn out to be false causing the system to malfunction.

In enterprise J2EE systems which generally have very large user base and involve interactions between heterogeneous systems, one of the general runtime issues reported is the system slowdown or system "hang". In such situations, the general trouble shooting pattern would be to analyze the Java thread dumps to isolate the threads which are causing the slow-down or hang. This article discusses the Java stack traces; anatomy of Java Threads and how to read thread dumps in general.

Exceptions and Stack Traces
All of us encounter and have encountered exceptions during our learning/development phases. Exception is the way Java reports a runtime error. Exceptions have two parts. Message and the stack trace. Message tells you what has gone wrong. Stack Traces provide a complete call flow through all the classes involved top-down as part of the runtime error.

The following is an example stack trace for an ArrayIndexOutOfBoundsException

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 4

at Test.run(Test.java:13)

at Test.<init>(Test.java:5)

at Test.main(Test.java:20)

In the above exception, the first line "Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 4" tells you that the JVM encountered the reported exception when it tried to access the value at array index 4. The java thread which encountered the exception is "main" thread.

Next let's read through the stack trace. The rule of thumb for exceptions is to read from the top line (message line) to understand what the exception is and keep going down to understand the call flow. In the above example, the call flow started at Test.java line number 20 (main() method) and then it called the constructor of the Test class. Constructors are represented with <init> in the stack traces. Then it moved on to the run() method in the Test class and encountered the reported exception at line number 13.

From the above stack trace, we can conclude that in Test.java, an attempt is made to read past the size of the Array.

The Java Thread Dump
Java Thread dump can be defined as a snapshot of all threads running inside a Java Virtual Machine (JVM) at a given point of time. A thread dump might contain a single thread or multiple threads. In multi threaded environments like J2EE application servers, there will be many threads and Thread Groups. Each of these threads will have its own call stack and will be performing a separate function at a given time. Thread Dump will provide the stack traces of all the JVM threads and much more information about a particular thread.

The Java VM Process and Java Threads
The Java Virtual Machine or JVM is an operating system level process. Java Threads are child processes or Light-Weight Processes (in Solaris terms) of the JVM process.

Generating Java Thread Dumps
Thread Dump is generated by sending a SIGQUIT signal to the JVM process. There are different ways of sending this signal to the process.

In Unix, use "kill -3 <pid>" where pid is the Process ID of the JVM.

In Windows, press CTRL+BREAK on the window where the JVM is running

Java Thread State
Each Java Thread can be in one of the four states during its lifetime.

Runnable - The thread is either running or ready to run when it gets its CPU turn. JRockit thread dumps refer to this state as ACTIVE.

Waiting on Monitor - The thread is either sleeping or waiting on an object for said period of time or waiting to be notified by another thread. This will happen when any of the sleep() methods on Thread object or wait() method on an Object is called.

For example, in WebLogc server the idle execute threads are in this condition and they wait till a socket reader thread notify them of some new work to be done. The stack trace will look like this:

"ExecuteThread: '2' for queue: 'weblogic.admin.RMI'" daemon prio=5 tid=0x1752F040 nid=0x180c in Object.wait() [1887f000..1887fd8c]

at java.lang.Object.wait(Native Method) waiting on <04134D98> (a weblogic.kernel.ExecuteThread)

at java.lang.Object.wait(Object.java:426)

at weblogic.kernel.ExecuteThread.waitForRequest(ExecuteThread.java:126)

locked <04134D98> (a weblogic.kernel.ExecuteThread)

at weblogic.kernel.ExecuteThread.run(ExecuteThread.java:145)

Some other versions of the JVM call this state as CW, Object.wait() (as above). JRockit refer to this as state as WAITING.

Waiting for Monitor Entry - The thread is waiting to get the lock for an object (some other thread may be holding the lock). This happens if two or more threads try to execute synchronized code. Note that the lock is always for an object and not for individual methods.

Sample stack trace of a thread in this condition:

"ExecuteThread: '24' for queue: 'DisplayExecuteQueue'" daemon prio=5 tid=0x5541b0 nid=0x3b waiting for monitor entry [49b7f000..49b7fc24]

at weblogic.cluster.replication.ReplicationManager.createSecondary (ReplicationManager.java:908)
- waiting to lock <6c4b9130> (a java.lang.Object)
at weblogic.cluster.replication.ReplicationManager.updateSecondary (ReplicationManager.java:715)
at weblogic.servlet.internal.session.ReplicatedSessionData.syncSession (ReplicatedSessonData.java:459)
- locked <6c408700> (a weblogic.servlet.internal.session.ReplicatedSessionData)
at weblogic.servlet.internal.session.ReplicatedSessionContext.sync (ReplicatedSessionContext.java:134)
- locked <6c408700> (aweblogic.servlet.internal.session.ReplicatedSessionData)
at weblogic.servlet.internal.ServletRequestImpl.syncSession (ServletRequestImpl.java:2418)
at weblogic.servlet.internal.WebAppServletContext.invokeServlet (WebAppServletContext.java:3137)
at weblogic.servlet.internal.ServletRequestImpl.execute (ServletRequestImpl.java:2544)
at weblogic.kernel.ExecuteThread.execute(ExecuteThread.java:153)
at weblogic.kernel.ExecuteThread.run(ExecuteThread.java:134)

In the above stacktrace, you can see that this thread holds the lock an object (6c408700) and waiting to lock another object (6c4b9130)

Some other JVMs may not give the object Ids with lock information in the stack trace. The same state may also be called as ‘MW'. JRockit refers to this state as LOCKED.

Anatomy of a Java Thread
To be able to read/analyze thread dump, it is important to understand various parts of a Thread dump. Let's take a simple Thread Stack example and read various parts of it.

"ExecuteThread: '1' " daemon prio=5 tid=0x628330 nid=0xf runnable [0xe4881000..0xe48819e0]

at com.vantive.vanjavi.VanJavi.VanCreateForm(Native Method)

at com.vantive.vanjavi.VanMain.open(VanMain.java:53)

at jsp_servlet._so.__newServiceOrder.printSOSection( __newServiceOrder.java:3547)

at jsp_servlet._so.__newServiceOrder._jspService (__newServiceOrder.java:5652)

at weblogic.servlet.jsp.JspBase.service(JspBase.java:27)

at weblogic.servlet.internal.ServletStubImpl.invokeServlet (ServletStubImpl.java:265)

at weblogic.servlet.internal.ServletStubImpl.invokeServlet (ServletStubImpl.java:200)

at weblogic.servlet.internal.WebAppServletContext.invokeServlet (WebAppServletContext.java:2495)

at weblogic.servlet.internal.ServletRequestImpl.execute (ServletRequestImpl.java:2204)

at weblogic.kernel.ExecuteThread.execute (ExecuteThread.java:139)

at weblogic.kernel.ExecuteThread.run(ExecuteThread.java:120)

In the above Thread Dump, the interesting part to is the first line. The rest of the stuff is nothing more than a general stack trace. Lets analyze the first line here

Execute Thread : 1 This indicates the name of the thread.

daemon Indicates that this Thread is a daemon thread

prio=5 The priority of this thread (default is 5)

tid The Java Thread Id (Unique identifier for this thread in the running JVM instance).

nid Native Identifier of the thread. Indicates LWP id in Solaris. Identifier for this process

at the OS level.

runnable Indicate the Thread state (See above)

[x..y] Indicates the range of addresses in the heap where this thread is executing.

The remaining part of the thread dump show the call flow. In this example, the thread (Execute Thread 1) is a OS daemon which is currently executing a native method vanCreateForm().

Putting Thread Dumps to Use
In this section I describe few use cases in which Thread dumps are very useful.

High CPU consumption
Diagnosis

The application seems to consume almost 100% CPU and the throughput has come down drastically. Starts to perform very poorly with high load on the CPU.

Thread Dump
All threads in the dump generally show one or more threads struck doing the same operation through all the thread dumps.

Solution

  • Take a series of thread dumps (around 5-7) for one particular call flow (a web form submit for example) before the call is completed.
  • Looking for "runnable" threads in the Thread Dump. If each such thread seems to move (the method call in each thread dump is different), then the thread is progressing and probably not a culprit. If the thread is executing the same method (same line number) through all the thread dumps, then it is almost for sure the culprit. Go to the code and do the code level analysis. You are almost certain to catch an issue there.

Low CPU consumption and Poor Response time
Diagnosis

This happens in a high I/O bound systems under high load. The CPU consumption is low with only few threads consuming reasonable CPU chunk. However the application response time will be very slow.

Thread Dump
Some or all runnable threads seem to struck performing an I/O operation like a File read/write or a database operation.

Solution
Understand the I/O operation profile of your application. Use caches where applicable to reduce DB interactions.

Application/Server Hang
Diagnosis

An application or an server JVM hosting the application will hang (become unresponsive).

Thread Dump

  • All runnable threads are struck doing the same operation through all the thread dumps taken. The server runs out of any more threads as all the runnable threads "never" complete their operation.
  • There might be many threads waiting for monitor entry. This can happen if a "runnable" thread is holding a lock on an object and never returns it while others are waiting for the same lock.

Solution

  • Check for dead lock. JVM generally detects a deadlock if it is simple enough (Thread A waiting for Thread B and vice versa). However, you need to understand the lock profile at the given moment to see if there is any complex deadlock situation involved.
  • Revisit the synchronized methods/block in the code. Reduce the size of synchronization area to the extent possible.
  • One issue could be too long a timeout while accessing a remote resource/component. Implement a reasonable timeout on the remote object clients so that the throw appropriate exception when a remote system does not respond in a reasonable amount of time.
  • If all the threads are waiting for a resource (like an EJB/DB connection), consider increasing the object pool size for those resources.

Tools
There are both commercial and open source tools available for thread dump analysis. One such tool is Samurai. It is a light weight open source tool which runs as a Java Web Start application as well as from your command prompt. For more information and documentation on Samurai, visit

http://yusuke.homeip.net/samurai/en/index.html

Conclusion
Maintaining J2EE enterprise application in a production environment is a tough job. As the business dynamics change, J2EE application environments change which might cause runtime instability in the production application. One of the primary factors that effect an running J2EE application is high load. While most systems are designed to be scalable, environmental limitations might cause them to become non-responsive.

Java Thread Dumps is an excellent mechanism to identify, diagnose, detect and resolve typical production issues. While application profiling and other mechanisms do exist, analyzing Java Thread dumps will provide us an clear and early understanding of popular production level issues there by saving time and helping us provide better user experience with the production applications.

More Stories By Shankar Itchapurapu

Shankar Itchapurapu is a software engineer at Oracle in India. He holds a Master's degree in Computer Applications. You can e-mail Shankar at shankar.itchapurapu@gmail.com.

@ThingsExpo Stories
BnkToTheFuture.com is the largest online investment platform for investing in FinTech, Bitcoin and Blockchain companies. We believe the future of finance looks very different from the past and we aim to invest and provide trading opportunities for qualifying investors that want to build a portfolio in the sector in compliance with international financial regulations.
A strange thing is happening along the way to the Internet of Things, namely far too many devices to work with and manage. It has become clear that we'll need much higher efficiency user experiences that can allow us to more easily and scalably work with the thousands of devices that will soon be in each of our lives. Enter the conversational interface revolution, combining bots we can literally talk with, gesture to, and even direct with our thoughts, with embedded artificial intelligence, whic...
Imagine if you will, a retail floor so densely packed with sensors that they can pick up the movements of insects scurrying across a store aisle. Or a component of a piece of factory equipment so well-instrumented that its digital twin provides resolution down to the micrometer.
In his keynote at 18th Cloud Expo, Andrew Keys, Co-Founder of ConsenSys Enterprise, provided an overview of the evolution of the Internet and the Database and the future of their combination – the Blockchain. Andrew Keys is Co-Founder of ConsenSys Enterprise. He comes to ConsenSys Enterprise with capital markets, technology and entrepreneurial experience. Previously, he worked for UBS investment bank in equities analysis. Later, he was responsible for the creation and distribution of life settle...
Product connectivity goes hand and hand these days with increased use of personal data. New IoT devices are becoming more personalized than ever before. In his session at 22nd Cloud Expo | DXWorld Expo, Nicolas Fierro, CEO of MIMIR Blockchain Solutions, will discuss how in order to protect your data and privacy, IoT applications need to embrace Blockchain technology for a new level of product security never before seen - or needed.
Leading companies, from the Global Fortune 500 to the smallest companies, are adopting hybrid cloud as the path to business advantage. Hybrid cloud depends on cloud services and on-premises infrastructure working in unison. Successful implementations require new levels of data mobility, enabled by an automated and seamless flow across on-premises and cloud resources. In his general session at 21st Cloud Expo, Greg Tevis, an IBM Storage Software Technical Strategist and Customer Solution Architec...
Nordstrom is transforming the way that they do business and the cloud is the key to enabling speed and hyper personalized customer experiences. In his session at 21st Cloud Expo, Ken Schow, VP of Engineering at Nordstrom, discussed some of the key learnings and common pitfalls of large enterprises moving to the cloud. This includes strategies around choosing a cloud provider(s), architecture, and lessons learned. In addition, he covered some of the best practices for structured team migration an...
No hype cycles or predictions of a gazillion things here. IoT is here. You get it. You know your business and have great ideas for a business transformation strategy. What comes next? Time to make it happen. In his session at @ThingsExpo, Jay Mason, an Associate Partner of Analytics, IoT & Cybersecurity at M&S Consulting, presented a step-by-step plan to develop your technology implementation strategy. He also discussed the evaluation of communication standards and IoT messaging protocols, data...
Coca-Cola’s Google powered digital signage system lays the groundwork for a more valuable connection between Coke and its customers. Digital signs pair software with high-resolution displays so that a message can be changed instantly based on what the operator wants to communicate or sell. In their Day 3 Keynote at 21st Cloud Expo, Greg Chambers, Global Group Director, Digital Innovation, Coca-Cola, and Vidya Nagarajan, a Senior Product Manager at Google, discussed how from store operations and ...
In his session at 21st Cloud Expo, Raju Shreewastava, founder of Big Data Trunk, provided a fun and simple way to introduce Machine Leaning to anyone and everyone. He solved a machine learning problem and demonstrated an easy way to be able to do machine learning without even coding. Raju Shreewastava is the founder of Big Data Trunk (www.BigDataTrunk.com), a Big Data Training and consulting firm with offices in the United States. He previously led the data warehouse/business intelligence and B...
"IBM is really all in on blockchain. We take a look at sort of the history of blockchain ledger technologies. It started out with bitcoin, Ethereum, and IBM evaluated these particular blockchain technologies and found they were anonymous and permissionless and that many companies were looking for permissioned blockchain," stated René Bostic, Technical VP of the IBM Cloud Unit in North America, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Conventi...
When shopping for a new data processing platform for IoT solutions, many development teams want to be able to test-drive options before making a choice. Yet when evaluating an IoT solution, it’s simply not feasible to do so at scale with physical devices. Building a sensor simulator is the next best choice; however, generating a realistic simulation at very high TPS with ease of configurability is a formidable challenge. When dealing with multiple application or transport protocols, you would be...
Smart cities have the potential to change our lives at so many levels for citizens: less pollution, reduced parking obstacles, better health, education and more energy savings. Real-time data streaming and the Internet of Things (IoT) possess the power to turn this vision into a reality. However, most organizations today are building their data infrastructure to focus solely on addressing immediate business needs vs. a platform capable of quickly adapting emerging technologies to address future ...
We are given a desktop platform with Java 8 or Java 9 installed and seek to find a way to deploy high-performance Java applications that use Java 3D and/or Jogl without having to run an installer. We are subject to the constraint that the applications be signed and deployed so that they can be run in a trusted environment (i.e., outside of the sandbox). Further, we seek to do this in a way that does not depend on bundling a JRE with our applications, as this makes downloads and installations rat...
Widespread fragmentation is stalling the growth of the IIoT and making it difficult for partners to work together. The number of software platforms, apps, hardware and connectivity standards is creating paralysis among businesses that are afraid of being locked into a solution. EdgeX Foundry is unifying the community around a common IoT edge framework and an ecosystem of interoperable components.
DX World EXPO, LLC, a Lighthouse Point, Florida-based startup trade show producer and the creator of "DXWorldEXPO® - Digital Transformation Conference & Expo" has announced its executive management team. The team is headed by Levent Selamoglu, who has been named CEO. "Now is the time for a truly global DX event, to bring together the leading minds from the technology world in a conversation about Digital Transformation," he said in making the announcement.
In this strange new world where more and more power is drawn from business technology, companies are effectively straddling two paths on the road to innovation and transformation into digital enterprises. The first path is the heritage trail – with “legacy” technology forming the background. Here, extant technologies are transformed by core IT teams to provide more API-driven approaches. Legacy systems can restrict companies that are transitioning into digital enterprises. To truly become a lead...
Digital Transformation (DX) is not a "one-size-fits all" strategy. Each organization needs to develop its own unique, long-term DX plan. It must do so by realizing that we now live in a data-driven age, and that technologies such as Cloud Computing, Big Data, the IoT, Cognitive Computing, and Blockchain are only tools. In her general session at 21st Cloud Expo, Rebecca Wanta explained how the strategy must focus on DX and include a commitment from top management to create great IT jobs, monitor ...
"Cloud Academy is an enterprise training platform for the cloud, specifically public clouds. We offer guided learning experiences on AWS, Azure, Google Cloud and all the surrounding methodologies and technologies that you need to know and your teams need to know in order to leverage the full benefits of the cloud," explained Alex Brower, VP of Marketing at Cloud Academy, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clar...
The IoT Will Grow: In what might be the most obvious prediction of the decade, the IoT will continue to expand next year, with more and more devices coming online every single day. What isn’t so obvious about this prediction: where that growth will occur. The retail, healthcare, and industrial/supply chain industries will likely see the greatest growth. Forrester Research has predicted the IoT will become “the backbone” of customer value as it continues to grow. It is no surprise that retail is ...