I have a Java class with native functions implemented in C++ which is called nz.ac.unitec.BigInteger. The native implementation of nz.ac.unitec.BigInteger is simple, just wraps the java.math.BigInteger and call its constructor and add, subtract, multiply ... functions. A field mNativeContext in nz.ac.unitec.BigInteger is used to store a global reference to a java.math.BigInteger object. The object has a finalizer that should destroy the global reference when the object is garbage collected so I am not leaking global references.
When I runned a simple test loop to create a number of nz.ac.unitec.BigInteger objects without explicitly releasing the created objects, a JNI error was reported as following log (LOG1). But if I release the created objects explicitly before leaving the test function, the stress test can be executed successfully.
Why is the JVM running out of global references when I have a finalizer to delete them when the objects are garbage collected?
LOG1:
F/art (10730): art/runtime/indirect_reference_table.cc:113] JNI ERROR (app bug): global reference table overflow (max=51200)
F/art (10730): art/runtime/indirect_reference_table.cc:113] global reference table dump:
F/art (10730): art/runtime/indirect_reference_table.cc:113] Last 10 entries (of 51200):
F/art (10730): art/runtime/indirect_reference_table.cc:113] 51199: 0x12e88790 java.math.BigInteger
F/art (10730): art/runtime/indirect_reference_table.cc:113] 51198: 0x12e85490 java.math.BigInteger
F/art (10730): art/runtime/indirect_reference_table.cc:113] 51197: 0x12e81790 java.math.BigInteger
F/art (10730): art/runtime/indirect_reference_table.cc:113] 51196: 0x12e7e760 java.math.BigInteger
F/art (10730): art/runtime/indirect_reference_table.cc:113] 51195: 0x12e7ab20 java.math.BigInteger
F/art (10730): art/runtime/indirect_reference_table.cc:113] 51194: 0x12e77790 java.math.BigInteger
F/art (10730): art/runtime/indirect_reference_table.cc:113] 51193: 0x12e73a90 java.math.BigInteger
F/art (10730): art/runtime/indirect_reference_table.cc:113] 51192: 0x12e71af0 java.math.BigInteger
F/art (10730): art/runtime/indirect_reference_table.cc:113] 51191: 0x12e6dd60 java.math.BigInteger
F/art (10730): art/runtime/indirect_reference_table.cc:113] 51190: 0x12e6b9a0 java.math.BigInteger
F/art (10730): art/runtime/indirect_reference_table.cc:113] Summary:
F/art (10730): art/runtime/indirect_reference_table.cc:113] 1456 of java.math.BigInteger (1456 unique instances)
F/art (10730): art/runtime/indirect_reference_table.cc:113] 2 of android.opengl.EGLDisplay (2 unique instances)
F/art (10730): art/runtime/indirect_reference_table.cc:113] 1889 of java.math.BigInteger (1889 unique instances)
F/art (10730): art/runtime/indirect_reference_table.cc:113] 1 of java.lang.String
F/art (10730): art/runtime/indirect_reference_table.cc:113] 27 of java.math.BigInteger (27 unique instances)
F/art (10730): art/runtime/indirect_reference_table.cc:113] 1 of java.lang.String
F/art (10730): art/runtime/indirect_reference_table.cc:113] 3771 of java.math.BigInteger (3771 unique instances)
F/art (10730): art/runtime/indirect_reference_table.cc:113] 1 of dalvik.system.PathClassLoader
F/art (10730): art/runtime/runtime.cc:284] Runtime aborting...
F/art (10730): art/runtime/runtime.cc:284] Aborting thread:
Calculator.java
package nz.ac.unitec.calculator;
public class MainActivity extends AppCompatActivity
{
protected void onCreate(Bundle savedInstanceState)
{
Random ra = new Random();
for(int i=0; i<6000000; ++i) {
testCreateFinalize(ra);
int m = ra.nextInt();
int n = 8;
int re = m + n;
Log.i("MainActivity", "re=" + re);
//BigInteger result = l.subtract(r);
}
private void testCreateFinalize(Random ra)
{
BigInteger l = new BigInteger("100", 10);
BigInteger r = new BigInteger("200", 10);
//l.release(); when adding this two code lines, the test is ok
//r.release();;
}
}
BigInteger.java
package nz.ac.unitec.mathutils;
public class BigInteger
{
static {
System.loadLibrary("BigInteger_jni");
native_init();
}
private long mNativeContext;
private BigInteger()
{
mNativeContext = 0;
native_setup();
}
public BigInteger(String val, int radix)
{
mNativeContext = 0;
native_setup_bystring(val, radix);
}
public void release() {
native_finalize();
}
protected void finalize() {
native_finalize();
}
private static native final void native_init();
private native final void native_setup();
private native final void native_setup_bystring(String val, int radix);
private native final void native_finalize();
public native String toString(int radix);
public native BigInteger add(BigInteger rval);
public native BigInteger multiply(BigInteger rval);
public native BigInteger subtract(BigInteger rval);
}
Native Code
static jobject getNativeBigInteger_l(JNIEnv* env, jobject thiz)
{
return reinterpret_cast<jobject> (env->GetLongField(thiz, fields.context)); //reinterpret_cast<jobject>
}
static void setNativeBigInteger_l(JNIEnv* env, jobject thiz, jobject bi)
{
env->SetLongField(thiz, fields.context, reinterpret_cast<jlong>(bi)); //reinterpret_cast<jlong>
}
JNIEXPORT void JNICALL Java_nz_ac_unitec_mathutils_BigInteger_native_1setup_1bystring
(JNIEnv *env, jobject thiz, jstring val, jint radix)
{
jclass cls = env->FindClass(gBuiltinClassBigInteger);
ALOGV("-Java_nz_ac_unitec_mathutils_BigInteger_native_1setup_1bystring env->FindClass(%s) return (%0x)",
gBuiltinClassBigInteger, cls);
if (cls == NULL) {
ALOGE("-Java_nz_ac_unitec_mathutils_BigInteger_native_1setup_1bystring env->FindClass(%s) return NULL", gBuiltinClassBigInteger);
return;
}
jmethodID constructor = env->GetMethodID(cls, "<init>", "(Ljava/lang/String;I)V");
if (constructor == NULL) {
env->DeleteLocalRef(cls);
ALOGE("-Java_nz_ac_unitec_mathutils_BigInteger_native_1setup_1bystring env->GetMethodID(%s) return NULL", "<init>");
return;
}
jobject jobj = env->NewObject(cls, constructor, val, radix);
ALOGV("-Java_nz_ac_unitec_mathutils_BigInteger_native_1setup_1bystring env->NewObject return (%0x)",
jobj);
if (NULL == jobj) {
env->DeleteLocalRef(cls);
ALOGE("-Java_nz_ac_unitec_mathutils_BigInteger_native_1setup_1bystring env->NewObject return NULL");
return;
}
jobject gjobj = env->NewGlobalRef(jobj);
ALOGV("-Java_nz_ac_unitec_mathutils_BigInteger_native_1setup_1bystring env->NewGlobalRef return (%0x)", gjobj);
setNativeBigInteger_l(env, thiz, gjobj);
env->DeleteLocalRef(jobj);
//env->DeleteLocalRef(cls);
}
JNIEXPORT void JNICALL
Java_nz_ac_unitec_mathutils_BigInteger_native_1finalize
(JNIEnv *env, jobject thiz)
{
ALOGV("+native_finalize");
jobject obj = getNativeBigInteger_l(env, thiz);
ALOGV("-Java_nz_ac_unitec_mathutils_BigInteger_native_1finalize getNativeBigInteger_l return (%0x)",
obj);
if (obj == NULL) {
ALOGE("-native_finalize getNativeBigInteger_l NULL");
return;
}
env->DeleteGlobalRef(obj);
setNativeBigInteger_l(env, thiz, NULL);
ALOGV("-native_finalize");
}
static JNINativeMethod gMethods[] = {
{"native_init", "()V",
(void *)Java_nz_ac_unitec_mathutils_BigInteger_native_1init},
{"native_setup", "()V",
(void *)Java_nz_ac_unitec_mathutils_BigInteger_native_1setup},
{"native_setup_bystring", "(Ljava/lang/String;I)V",
(void*)Java_nz_ac_unitec_mathutils_BigInteger_native_1setup_1bystring},
{"native_finalize", "()V",
(void *)Java_nz_ac_unitec_mathutils_BigInteger_native_1finalize},
{"add", "(Lnz/ac/unitec/mathutils/BigInteger;)Lnz/ac/unitec/mathutils/BigInteger;",
(void *)Java_nz_ac_unitec_mathutils_BigInteger_add},
{"multiply","(Lnz/ac/unitec/mathutils/BigInteger;)Lnz/ac/unitec/mathutils/BigInteger;",
(void *)Java_nz_ac_unitec_mathutils_BigInteger_multiply},
{"subtract","(Lnz/ac/unitec/mathutils/BigInteger;)Lnz/ac/unitec/mathutils/BigInteger;",
(void *)Java_nz_ac_unitec_mathutils_BigInteger_subtract},
{"toString","(I)Ljava/lang/String;",
(void *)Java_nz_ac_unitec_mathutils_BigInteger_toString}
};
int register_Java_nz_ac_unitec_mathutils_BigInteger(JNIEnv *env)
{
return jniRegisterNativeMethods(env, gClassBigInteger, gMethods, NELEM(gMethods));
}
Your code is failing because you have too many global references to objects that have not been released back into the memory pool. The global reference table has a maximum size to help you catch memory leaks and prevent your program from running out of memory. The log message you pasted tells you what those objects are: java.math.BigInteger.
If you look at your implementation of native_setup_bystring you can see that you're creating a global reference to a new BigInteger object:
jobject gjobj = env->NewGlobalRef(jobs);
Global references are not automatically garbage collected [1][2] so you need to explicitly delete them, which is what you've found in your testing.
The problem with your approach is that you're storing a reference to the internal object using a direct memory reference into the heap (your long mNativeContext field). This approach is not a good one because you're preventing the the JVM from managing the garbage collection of your BigIntegers. A better approach would be to avoid using a global reference at all, and store an object reference instead of a long. If you do this then the JVM will be able to automatically collect all of the objects you're allocating.