To obtain thread dumps, you can collect multiple times through jstack, kill -3, JConsole or Spring Boot Actuator; 2. In the thread state, RUNNABLE may correspond to high CPU or infinite loops, BLOCKED indicates lock competition, WAITING/TIMED_WAITING is a waiting state, and you need to pay attention to exception accumulation; 3. Deadlock will be clearly indicated by jstack, which is manifested as a loop waiting lock, and should be solved by unified lock sequence or reduced lock granularity; 4. High CPU threads need to combine top and hexadecimal conversion positioning to check whether there are regular backtracking, serialization and other time-consuming operations in the call stack; 5. A large number of BLOCKED threads pointing to the same lock object, indicating that lock competition is serious, and tasks may accumulate due to too large synchronization range or too small thread pool; 6. You can use jstack command line tool or IBM TMDA, fastthread.io and other tools for automated analysis and visualization; 7. The actual analysis should collect at least 3 dumps with a 10-second interval, and check deadlock, blocking thread, high CPU thread, WAITING rationality, exception loops and system thread activity in turn, and finally establish the association of "status → stack → lock → code" to locate the root cause of the problem.
Analyzing thread dumps for Java applications is a key means to diagnose performance problems, deadlocks, high CPU usage and application hangs. Thread dump provides snapshots of all threads of the JVM at a certain moment, including the status of each thread, call stack, lock information, etc. By analyzing this information, the root cause of the problem can be located.
The following are the core points and practical steps for thread dump analysis:
1. How to get thread dump
Common ways include:
-
Use
jstacktool :jstack <pid> > threaddump.txt
It can be collected multiple times (5~10 seconds apart) to facilitate observation of thread state changes.
Send signals using
kill -3(suitable for applications that configure-XX: PrintConcurrentLocksand-XX: PrintGC):kill -3 <pid>
The output is printed to the application's standard output or log file.
Remotely connect to JVM export via JConsole or VisualVM graphical tools .
App internal triggers (such as
/actuator/threaddumpendpoint of Spring Boot Actuator).
2. Interpretation of thread state: key state and its meaning
Common states of threads in dumps are:
- RUNNABLE : is running or ready to run. It may be consuming CPU, so you need to pay attention to whether its call stack is stuck in infinite loops or frequent GC.
- BLOCKED : Wait for entering synchronized block/method. Commonly in lock competition.
- WAITING : Call
wait(),join(),park(), etc., and wait infinitely to be awakened. - TIMED_WAITING : Wait with timeout, such as
sleep(1000)andwait(500). - NEW / TERMINATED : Newly created or ended, generally no attention is required.
?? Note: A large number of
BLOCKEDthreads usually mean lock bottlenecks; continuousRUNNABLEand high CPU, which may correspond to hotspot code.
3. Frequently Asked Question Pattern Recognition
? Deadlock (Deadlock)
Search in the dump:
Found one Java-level deadlock:
jstack will directly prompt the deadlock threads and their respective locks and waiting locks.
Sample snippet:
"Thread-1" waiting to lock monitor 0x00007f8a8c0b5b50 (object 0x00000007d0a892c8, a java.lang.Object) waiting for monitor 0x00007f8a8c0b4d50 (object 0x00000007d0a892d8, a java.lang.Object) held by "Thread-2" "Thread-2" waiting to lock monitor 0x00007f8a8c0b4d50 (object 0x00000007d0a892d8, a java.lang.Object) waiting for monitor 0x00007f8a8c0b5b50 (object 0x00000007d0a892c8, a java.lang.Object) held by "Thread-1"
? Solution : Unify the lock order, use tryLock , and reduce the lock granularity.
? High CPU usage
- Use
top -H -p <pid>to find out the thread ID (LWP) of the high CPU. - Convert LWP to hexadecimal (such as
printf "%x\n" 1234→4d2). - Search for threads with
nid=0x4d2in thread dump. - Check its stack to confirm whether it is performing intensive calculations, regular matching, serialization, GC-related operations, etc.
Common traps:
String.split()uses regularity, complex regularity may cause backtracking explosions.
? Thread blocking or application unresponsive
Check whether there are a large number of threads in BLOCKED state and the stack points to the same lock object.
For example:
"worker-thread-5" - Thread t@50 java.lang.Thread.State: BLOCKED (on object monitor) at com.example.Service.doWork(Service.java:123) - waiting to lock <0x00000007d0a892c8> (a java.lang.Object)
Multiple threads are waiting for the same object to lock → lock competition is serious, consider optimizing the synchronization range or using concurrent containers.
? Thread pool exhaustion or task accumulation
Check the thread naming mode (such as pool-1-thread-1 ). If all threads are executing long-term tasks, it means that the task design is unreasonable or the thread pool configuration is too small.
It may also be manifested as a large number of threads in WAITING state, waiting for the task to be enqueued ( ThreadPoolExecutor.getTask ).
4. Recommended analysis tools
- jstack grep/sed/awk : Basic but efficient, suitable for automated scripts.
- IBM Thread and Monitor Dump Analyzer (TMDA) : Automatically identify deadlocks and thread status statistics.
- fastthread.io : Upload dump files and generate visual reports (support multiple comparisons).
- JProfiler / YourKit : A commercial tool that can deeply analyze thread behavior and performance hotspots.
Tips: Compare the dumps at multiple points in time and observe whether some threads always stay on the same line of code → may be stuck.
5. List of practical analysis steps
- [ ] Get at least 3 thread dumps with 10 seconds intervals.
- [ ] Check whether there is
deadlockprompt. - [ ] Search for
BLOCKEDthreads, count the number and stack commonality. - [ ] Locate high CPU threads (combined with system commands).
- [ ] Check whether
WAITINGthread is reasonable (such as timed tasks, connection pool waiting). - [ ] Check whether there is a custom thread dead loop or exception state.
- [ ] Pay attention to whether the system threads such as
Finalizer,GC task,Common-Cleanerare extremely active.
Basically that's it. Thread dump analysis is not complicated, but requires patience and understanding of concurrent programming. The key is to create an association chain of "Status → Stack → Lock → Code". Practice a few more times and you can quickly locate thread-level problems in most Java applications.
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