Welcome!

Java IoT Authors: Sanjeev Khurana, Elizabeth White, Carmen Gonzalez, Patrick Hubbard, Liz McMillan

Related Topics: Java IoT, Weblogic, Containers Expo Blog, Recurring Revenue

Java IoT: Article

Dealing with Application Deadlocks

Detecting and reporting deadlocks with the WebLogic Server

A deadlock is a situation where two or more threads are blocked while waiting to obtain locks that some of the other threads in the deadlock are holding. Deadlocks can occur when multiple threads need the same locks, at the same time, but obtain them in different order. For instance, if thread 1 locks A, and tries to lock B, and thread 2 has already locked B, and tries to lock A, a deadlock arises. Thread 1 can never get B, and thread 2 can never get A. In addition, neither of them will ever know. They will remain blocked on each of their objects, A and B, forever.

Deadlock Detection
The most common way of detecting whether your application is deadlocked is by analyzing thread dumps of the Java Virtual Machine while its in the state of appearing hung. A thread dump is a snapshot of the virtual machine's current state, including stack traces for each Java thread.

The thread dump outputs the state of each thread, which may be one of the following:

  • NEW: This state represents a new thread that is not yet started.
  • RUNNABLE: This state represents a thread that is executing in the underlying JVM. Here executing in the JVM doesn't mean that the thread is always executing in the OS as well - it may wait for a resource from the operating system like the processor while being in this state.
  • BLOCKED: This state represents a thread that has been blocked and is waiting for a moniotor to enter/re-enter a synchronized block/method. A thread gets into this state after calling the Object.wait method.
  • WAITING: This state represnts a thread in the waiting state. This wait is over only when some other thread performs some appropriate action. A thread can get into this state either by calling - Object.wait (without timeout), Thread.join (without timeout), or LockSupport.park methods.
  • TIMED_WAITING: This state represents a thread that is required to wait at max for a specified time limit. A thread can get into this state by calling any of these methods: Thread.sleep, Object.wait (with timeout specified), Thread.join (with timeout specified), LockSupport.parkNanos, and LockSupport.parkUntil
  • TERMINATED: This state reprents a thread that has completed its execution either by returning from the run() method after completing the execution OR by throwing an exception that propagated from the run() method and hence caused the termination of the thread.

A thread that appears as "java.lang.Thread.State: BLOCKED (on object monitor)" in the stacktrace indicates that it's waiting to enter a synchronized block and acquire a Java monitor. Alternatively, the code may call an Object.wait method every time the lock is not acquired, causing the threads to appear as "java.lang.Thread.State: WAITING" in the stacktrace.

A deadlock is detected if a complete circle of such thrads can be seen in the thread dump where two or more threads are blocked/waiting, with each blocked/waiting thread waiting to acquire a lock on an object that has been locked by another blocked/waiting thread.

Example :

"Thread-1" prio=6 tid=0x02a99c00 nid=0xee0 waiting for monitor entr...
java.lang.Thread.State: BLOCKED (on object monitor)
at SimpleDeadLock$Thread2.run(SimpleDeadLock.java:37)
- waiting to lock <0x229bd238> (a java.lang.Object)
- locked <0x229bd240> (a java.lang.Object)

"Thread-0" prio=6 tid=0x02a99000 nid=0xefc waiting for monitor entr...
java.lang.Thread.State: BLOCKED (on object monitor)
at SimpleDeadLock$Thread1.run(SimpleDeadLock.java:24)
- waiting to lock <0x229bd240> (a java.lang.Object)
- locked <0x229bd238> (a java.lang.Object)

In the above thread dump, it can be seen that "Thread-0" and "Thread-1" are in a "BLOCKED" state. "Thread-0" holds a lock on object "0x229bd238" and is waiting to acquire a lock on object "0x229bd240", while at the same time "Thread-1" holds a lock on object "0x229bd240" and is waiting to acquire a lock on object "0x229bd238". Since each of the threads has acquired a lock on an object that the other thread needs in order to complete its operation, they will wait in the same state infinitely, causing the application to hang.

Acquiring Thread Dumps
A thread dump of a running JVM may be taken in various ways, a few of the methods are listed below:

  • by sending a SIGQUIT signal to the JVM

kill -3 pid > threadDumpFilename 2>&1

  • by using WebLogic scripting tool
    export JAVA_HOME=<java home>
    export WL_HOME=<WebLogic home>

    cd WL_HOME/server/bin
    . ./setWLSEnv.sh
    java WebLogic.WLST

    connect("username","password","t3://<url>:<port>")
    cd ('Servers')
    cd ('<server name>')
    threadDump()

 

  • by using WebLogic.Admin utility (deprecated)
    export JAVA_HOME=<java home>
    export WL_HOME=<WebLogic home>

    cd WL_HOME/server/bin
    . ./setWLSEnv.sh

java WebLogic.Admin <url>:<port> -username <username> -password <password> THREAD_DUMP

  • by using WebLogic admin console

Login to AdminConsole->Server -> Monitoring -> Threads

  • by using Jrockit command line utility (Jrockit JVM)
    export JAVA_HOME=<java home>
    export WL_HOME=<WebLogic home>

    cd WL_HOME/server/bin
    . ./
    setWLSEnv.sh

jrcmd <pid> print_threads

· by using the Jstack utility (HotSpot JVM)
export JAVA_HOME=<java home>
export WL_HOME=<WebLogic home>

cd WL_HOME/server/bin
. ./setWLSEnv.sh

jstack <pid>

More Stories By Sanat Vij

Sanat Vij is a professional software engineer currently working at CenturyLink. He has vast experience in developing high availability applications, configuring application servers, JVM profiling and memory management. He specializes in performance tuning of applications, reducing response times, and increasing stability.

Comments (0)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


@ThingsExpo Stories
In his General Session at 17th Cloud Expo, Bruce Swann, Senior Product Marketing Manager for Adobe Campaign, explored the key ingredients of cross-channel marketing in a digital world. Learn how the Adobe Marketing Cloud can help marketers embrace opportunities for personalized, relevant and real-time customer engagement across offline (direct mail, point of sale, call center) and digital (email, website, SMS, mobile apps, social networks, connected objects).
With the introduction of IoT and Smart Living in every aspect of our lives, one question has become relevant: What are the security implications? To answer this, first we have to look and explore the security models of the technologies that IoT is founded upon. In his session at @ThingsExpo, Nevi Kaja, a Research Engineer at Ford Motor Company, will discuss some of the security challenges of the IoT infrastructure and relate how these aspects impact Smart Living. The material will be delivered i...
SYS-CON Events announced today that Hitrons Solutions will exhibit at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. Hitrons Solutions Inc. is distributor in the North American market for unique products and services of small and medium-size businesses, including cloud services and solutions, SEO marketing platforms, and mobile applications.
Your homes and cars can be automated and self-serviced. Why can't your storage? From simply asking questions to analyze and troubleshoot your infrastructure, to provisioning storage with snapshots, recovery and replication, your wildest sci-fi dream has come true. In his session at @DevOpsSummit at 20th Cloud Expo, Dan Florea, Director of Product Management at Tintri, will provide a ChatOps demo where you can talk to your storage and manage it from anywhere, through Slack and similar services ...
SYS-CON Events announced today that CA Technologies has been named “Platinum Sponsor” of SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY, and the 21st International Cloud Expo®, which will take place October 31-November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. CA Technologies helps customers succeed in a future where every business – from apparel to energy – is being rewritten by software. From ...
SYS-CON Events announced today that Cloudistics, an on-premises cloud computing company, has been named “Bronze Sponsor” of SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Cloudistics delivers a complete public cloud experience with composable on-premises infrastructures to medium and large enterprises. Its software-defined technology natively converges network, storage, compute, virtualization, and management into a ...
Keeping pace with advancements in software delivery processes and tooling is taxing even for the most proficient organizations. Point tools, platforms, open source and the increasing adoption of private and public cloud services requires strong engineering rigor - all in the face of developer demands to use the tools of choice. As Agile has settled in as a mainstream practice, now DevOps has emerged as the next wave to improve software delivery speed and output. To make DevOps work, organization...
My team embarked on building a data lake for our sales and marketing data to better understand customer journeys. This required building a hybrid data pipeline to connect our cloud CRM with the new Hadoop Data Lake. One challenge is that IT was not in a position to provide support until we proved value and marketing did not have the experience, so we embarked on the journey ourselves within the product marketing team for our line of business within Progress. In his session at @BigDataExpo, Sum...
DevOps is often described as a combination of technology and culture. Without both, DevOps isn't complete. However, applying the culture to outdated technology is a recipe for disaster; as response times grow and connections between teams are delayed by technology, the culture will die. A Nutanix Enterprise Cloud has many benefits that provide the needed base for a true DevOps paradigm.
SYS-CON Events announced today that Ocean9will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Ocean9 provides cloud services for Backup, Disaster Recovery (DRaaS) and instant Innovation, and redefines enterprise infrastructure with its cloud native subscription offerings for mission critical SAP workloads.
With major technology companies and startups seriously embracing Cloud strategies, now is the perfect time to attend @CloudExpo | @ThingsExpo, June 6-8, 2017, at the Javits Center in New York City, NY and October 31 - November 2, 2017, Santa Clara Convention Center, CA. Learn what is going on, contribute to the discussions, and ensure that your enterprise is on the right path to Digital Transformation.
SYS-CON Events announced today that T-Mobile will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. As America's Un-carrier, T-Mobile US, Inc., is redefining the way consumers and businesses buy wireless services through leading product and service innovation. The Company's advanced nationwide 4G LTE network delivers outstanding wireless experiences to 67.4 million customers who are unwilling to compromise on ...
SYS-CON Events announced today that SoftLayer, an IBM Company, has been named “Gold Sponsor” of SYS-CON's 18th Cloud Expo, which will take place on June 7-9, 2016, at the Javits Center in New York, New York. SoftLayer, an IBM Company, provides cloud infrastructure as a service from a growing number of data centers and network points of presence around the world. SoftLayer’s customers range from Web startups to global enterprises.
SYS-CON Events announced today that CrowdReviews.com has been named “Media Sponsor” of SYS-CON's 20th International Cloud Expo, which will take place on June 6–8, 2017, at the Javits Center in New York City, NY. CrowdReviews.com is a transparent online platform for determining which products and services are the best based on the opinion of the crowd. The crowd consists of Internet users that have experienced products and services first-hand and have an interest in letting other potential buyers...
The taxi industry never saw Uber coming. Startups are a threat to incumbents like never before, and a major enabler for startups is that they are instantly “cloud ready.” If innovation moves at the pace of IT, then your company is in trouble. Why? Because your data center will not keep up with frenetic pace AWS, Microsoft and Google are rolling out new capabilities In his session at 20th Cloud Expo, Don Browning, VP of Cloud Architecture at Turner, will posit that disruption is inevitable for c...
SYS-CON Events announced today that Infranics will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Since 2000, Infranics has developed SysMaster Suite, which is required for the stable and efficient management of ICT infrastructure. The ICT management solution developed and provided by Infranics continues to add intelligence to the ICT infrastructure through the IMC (Infra Management Cycle) based on mathemat...
SYS-CON Events announced today that SD Times | BZ Media has been named “Media Sponsor” of SYS-CON's 20th International Cloud Expo, which will take place on June 6–8, 2017, at the Javits Center in New York City, NY. BZ Media LLC is a high-tech media company that produces technical conferences and expositions, and publishes a magazine, newsletters and websites in the software development, SharePoint, mobile development and commercial UAV markets.
Now that the world has connected “things,” we need to build these devices as truly intelligent in order to create instantaneous and precise results. This means you have to do as much of the processing at the point of entry as you can: at the edge. The killer use cases for IoT are becoming manifest through AI engines on edge devices. An autonomous car has this dual edge/cloud analytics model, producing precise, real-time results. In his session at @ThingsExpo, John Crupi, Vice President and Eng...
SYS-CON Events announced today that Telecom Reseller has been named “Media Sponsor” of SYS-CON's 20th International Cloud Expo, which will take place on June 6–8, 2017, at the Javits Center in New York City, NY. Telecom Reseller reports on Unified Communications, UCaaS, BPaaS for enterprise and SMBs. They report extensively on both customer premises based solutions such as IP-PBX as well as cloud based and hosted platforms.
In his General Session at 16th Cloud Expo, David Shacochis, host of The Hybrid IT Files podcast and Vice President at CenturyLink, investigated three key trends of the “gigabit economy" though the story of a Fortune 500 communications company in transformation. Narrating how multi-modal hybrid IT, service automation, and agile delivery all intersect, he will cover the role of storytelling and empathy in achieving strategic alignment between the enterprise and its information technology.