If a class refer to its subclasses in its static initializers or in static fields, this references can cause JVM-level deadlocks in multithreaded environment, when one thread tries to load superclass and another thread tries to load subclass at the same time.
See: https://bugs.openjdk.java.net/browse/JDK-8037567
The following demo code can reproduce this deadlock
public class ClassInitDeadLock {
public static void main(String[] args) throws InterruptedException {
Thread threadA = new Thread(new Runnable() {
@Override
public void run() {
// trigger supper class initialize
PascalCaseStrategy strategy = PropertyNamingStrategy.PASCAL_CASE_TO_CAMEL_CASE;
}
}, "Thread-A");
Thread threadB = new Thread(new Runnable() {
@Override
public void run() {
// trigger sub class initialize
PascalCaseStrategy strategy = new PascalCaseStrategy();
}
}, "Thread-B");
threadA.start();
Thread.sleep(100);
threadB.start();
}
public static class PropertyNamingStrategy {
static {
try {
// sleep 1s for deadlock reproduction
Thread.sleep(1000);
} catch (InterruptedException e) {
// ignore
}
}
public static final PascalCaseStrategy PASCAL_CASE_TO_CAMEL_CASE = new PascalCaseStrategy();
}
public static class PascalCaseStrategy extends PropertyNamingStrategy {
}
}When the demo program ClassInitDeadLock started, it could not exit automatically. Thread-A holding PropertyNamingStrategy class , and waiting PascalCaseStrategy class initialize, while Thread-B holding PascalCaseStrategy class, and waiting PropertyNamingStrategy class initialize, deadlock occurred!
jstack of these two thread:
"Thread-A" #11 prio=5 os_prio=31 tid=0x00007fa8c29d8000 nid=0xa803 in Object.wait() [0x000070000a4c4000]
java.lang.Thread.State: RUNNABLE
at ClassInitDeadLock$PropertyNamingStrategy.<clinit>(ClassInitDeadLock.java:35)
at ClassInitDeadLock$1.run(ClassInitDeadLock.java:8)
at java.lang.Thread.run(Thread.java:748)
"Thread-B" #12 prio=5 os_prio=31 tid=0x00007fa8c4a9b800 nid=0x5603 in Object.wait() [0x000070000a5c7000]
java.lang.Thread.State: RUNNABLE
at ClassInitDeadLock$2.run(ClassInitDeadLock.java:16)
at java.lang.Thread.run(Thread.java:748)
In the actual environment, PropertyNamingStrategy and its subclass defined in jackson-databind initialize very fast, so It's hard to produce the scene that multithread load superclass and subclass at the same time.
I happened to meet this deadlock recently, It turn out to be JsonErrorUnmarshaller.java#L37 defined in aws-java-sdk-core, trying to initialize subclass PascalCaseStrategy, at the same time, another thread is initilizing superclass PropertyNamingStrategy
private static final ObjectMapper MAPPER = new ObjectMapper().configure(
DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false).setPropertyNamingStrategy(
new PascalCaseStrategy());
If a class refer to its subclasses in its static initializers or in static fields, this references can cause JVM-level deadlocks in multithreaded environment, when one thread tries to load superclass and another thread tries to load subclass at the same time.
See: https://bugs.openjdk.java.net/browse/JDK-8037567
The following demo code can reproduce this deadlock
When the demo program
ClassInitDeadLockstarted, it could not exit automatically.Thread-AholdingPropertyNamingStrategyclass , and waitingPascalCaseStrategyclass initialize, whileThread-BholdingPascalCaseStrategyclass, and waitingPropertyNamingStrategyclass initialize, deadlock occurred!jstack of these two thread:
In the actual environment,
PropertyNamingStrategyand its subclass defined in jackson-databind initialize very fast, so It's hard to produce the scene that multithread load superclass and subclass at the same time.I happened to meet this deadlock recently, It turn out to be JsonErrorUnmarshaller.java#L37 defined in
aws-java-sdk-core, trying to initialize subclassPascalCaseStrategy, at the same time, another thread is initilizing superclassPropertyNamingStrategy