JNI Warnings in OpenJDK 8 from JVM

[Adam Retter]

Hi there,

I have been working on RocksJava (https://github.com/facebook/rocksdb)
which is a combination of C++ JNI and Java 7.

When running this (make jcheck) with the java argument `-Xcheck:jni`
on the Java 8 VM, we noticed lots of warnings along the lines of:

WARNING in native method: JNI call made without checking exceptions
when required to from CallObjectMethod
WARNING in native method: JNI call made without checking exceptions
when required to from CallVoidMethod
WARNING in native method: JNI call made without checking exceptions
when required to from CallStaticVoidMethod
...

We modified our code to call `env->ExceptionCheck()` appropriately (in
this branch: https://github.com/adamretter/rocksdb/tree/java8), and
the vast majority of these warnings have disappeared. However we do
still seem to be getting several warnings (3 distinct warnings
repeated), these if I am not mistaken are actually being omitted from
code within the JVM itself rather than RocksJava code. The warnings
with traces look like:


WARNING in native method: JNI call made without checking exceptions
when required to from CallStaticVoidMethod
at java.lang.Class.isInstance(Native Method)
at org.junit.runners.model.TestClass.getAnnotatedFieldValues(TestClass.java:231)
at org.junit.runners.ParentRunner.classRules(ParentRunner.java:255)
at org.junit.runners.ParentRunner.withClassRules(ParentRunner.java:244)
at org.junit.runners.ParentRunner.classBlock(ParentRunner.java:194)
at org.junit.runners.ParentRunner.run(ParentRunner.java:362)
at org.junit.runners.Suite.runChild(Suite.java:128)
at org.junit.runners.Suite.runChild(Suite.java:27)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
at org.junit.runner.JUnitCore.run(JUnitCore.java:105)
at org.junit.runner.JUnitCore.run(JUnitCore.java:94)
at org.rocksdb.test.RocksJunitRunner.main(RocksJunitRunner.java:61)


WARNING in native method: JNI call made without checking exceptions
when required to from CallStaticVoidMethod
at java.lang.Class.getSuperclass(Native Method)
at sun.reflect.Reflection.isSubclassOf(Reflection.java:247)
at sun.reflect.ReflectionFactory.newConstructorAccessor(ReflectionFactory.java:194)
at java.lang.reflect.Constructor.acquireConstructorAccessor(Constructor.java:460)
at java.lang.reflect.Constructor.newInstance(Constructor.java:420)
at org.junit.runners.BlockJUnit4ClassRunner.createTest(BlockJUnit4ClassRunner.java:217)
at org.junit.runners.BlockJUnit4ClassRunner$1.runReflectiveCall(BlockJUnit4ClassRunner.java:266)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.BlockJUnit4ClassRunner.methodBlock(BlockJUnit4ClassRunner.java:263)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runners.Suite.runChild(Suite.java:128)
at org.junit.runners.Suite.runChild(Suite.java:27)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
at org.junit.runner.JUnitCore.run(JUnitCore.java:105)
at org.junit.runner.JUnitCore.run(JUnitCore.java:94)
at org.rocksdb.test.RocksJunitRunner.main(RocksJunitRunner.java:61)


WARNING in native method: JNI call made without checking exceptions
when required to from CallStaticVoidMethod
at java.lang.ClassLoader.findBootstrapClass(Native Method)
at java.lang.ClassLoader.findBootstrapClassOrNull(ClassLoader.java:1015)
at java.lang.ClassLoader.loadClass(ClassLoader.java:413)
- locked <0x000000076ab00380> (a java.lang.Object)
at java.lang.ClassLoader.loadClass(ClassLoader.java:411)
- locked <0x000000076ab002d0> (a java.lang.Object)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at org.junit.runner.notification.RunNotifier.fireTestRunFinished(RunNotifier.java:100)
at org.junit.runner.JUnitCore.run(JUnitCore.java:138)
at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
at org.junit.runner.JUnitCore.run(JUnitCore.java:105)
at org.junit.runner.JUnitCore.run(JUnitCore.java:94)
at org.rocksdb.test.RocksJunitRunner.main(RocksJunitRunner.java:61)


It seems I am not the only developer noticing this unexpected
behaviour with Java 8, here is a bug report for DropBox -
https://github.com/dropbox/djinni/issues/152#issuecomment-157220326

The test cases I have collected that reproduce some of these warnings
(weirdly I struggled to reproduce exactly the warnings I see from
RocksJava) look like:

Test Case 1
----------------
$ echo 'public class Foo { public static void main(String[] args)
throws Exception { } }' > Foo.java
$ javac Foo.java
$ java -Xcheck:jni Foo

WARNING in native method: JNI call made without checking exceptions
when required to from CallStaticObjectMethod
WARNING in native method: JNI call made without checking exceptions
when required to from CallObjectMethod

Test Case 2
----------------
$ echo 'public class Foo2 { public static void main(String[] args)
throws Exception {
System.runFinalization(); System.runFinalization(); } }' > Foo2.java
$ javac Foo2.java
$ java -Xcheck:jni Foo2

WARNING in native method: JNI call made without checking exceptions
when required to from CallStaticObjectMethod
WARNING in native method: JNI call made without checking exceptions
when required to from CallObjectMethod
WARNING in native method: JNI call made without checking exceptions
when required to from CallStaticVoidMethod
at java.lang.Runtime.runFinalization0(Native Method)
at java.lang.Runtime.runFinalization(Runtime.java:712)
at java.lang.System.runFinalization(System.java:1015)
at Foo.main(Foo.java:1)

NOTE - you need the `System.runFinalization();` twice to cause the last warning.


Test Case 3
----------------
$ echo 'import java.net.NetworkInterface; public class Foo3 { public
static void main(String[] args) throws Exception {
NetworkInterface.getNetworkInterfaces(); } }' > Foo3.java
$ javac Foo3.java
$ java -Xcheck:jni Foo3

WARNING in native method: JNI call made without checking exceptions
when required to from CallStaticObjectMethod
WARNING in native method: JNI call made without checking exceptions
when required to from CallObjectMethod
WARNING: JNI local refs: 33, exceeds capacity: 32
at java.net.NetworkInterface.getAll(Native Method)
at java.net.NetworkInterface.getNetworkInterfaces(NetworkInterface.java:343)
at java.net.DefaultInterface.chooseDefaultInterface(DefaultInterface.java:67)
at java.net.DefaultInterface.<clinit>(DefaultInterface.java:46)
at java.net.NetworkInterface.<clinit>(NetworkInterface.java:65)
at Foo3.main(Foo3.java:1)
WARNING: JNI local refs: 33, exceeds capacity: 32
at java.net.NetworkInterface.getAll(Native Method)
at java.net.NetworkInterface.getNetworkInterfaces(NetworkInterface.java:343)
at Foo3.main(Foo3.java:1)


I have searched the OpenJDK and OracleJDK bug trackers but could not
find an issue that quite covers this. For reference I have tested all
of the above on Oracle JDK 1.8.0_121-b13 and Azul Zulu OpenJDK
8.20.0.5-jdk8.0.121 on Apple macOS 10.12.3.

I would be interested to find out more about what is causing this
issue and how to resolve it.

Thanks Adam.


-- 
Adam Retter

skype: adam.retter
tweet: adamretter
http://www.adamretter.org.uk