PropertyNamingStrategy class initialization depends on its subclass, this can lead to class loading deadlock #2715
Description
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());