为什么需要释放在JNI本机函数中创建的全局引用？

Question

我有一个Java类，具有使用C
++实现的本机函数，称为nz.ac.unitec.BigInteger。的本机实现nz.ac.unitec.BigInteger很简单，只是包裹java.math.BigInteger，并调用它的构造函数和add，subtract，multiply…功能。一个字段mNativeContext中nz.ac.unitec.BigInteger用于存储的全局引用一个java.math.BigInteger对象。该对象具有终结器，该终结器应该在垃圾回收对象时破坏全局引用，因此我不会泄漏全局引用。

当我运行一个简单的测试循环以创建多个nz.ac.unitec.BigInteger对象而不显式释放所创建的对象时，在日志（LOG1）之后报告了JNI错误。但是，如果我在离开测试功能之前显式释放创建的对象，则压力测试可以成功执行。

当我有终结器要在垃圾回收对象中删除它们时，为什么JVM的全局引用用完了？

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:

计算器.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);

}

本机代码

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));
}

Answer 1

您的代码失败，因为您对尚未释放回内存池中的对象的全局引用过多。全局引用表的最大大小可帮助您捕获内存泄漏并防止程序内存不足。您粘贴的日志消息告诉您这些对象是什么：java.math.BigInteger。

如果查看您的实现，native_setup_bystring可以看到您正在创建对新BigInteger对象的全局引用：

jobject gjobj = env->NewGlobalRef(jobs);

全局引用不会自动垃圾收集[1]

[2]，因此您需要显式删除它们，这是在测试中找到的。

方法的问题在于，您正在使用对堆（您的long mNativeContext字段）中的直接内存引用来存储对内部对象的引用
。这种方法不是一个好方法，因为您要阻止JVM管理BigIntegers的垃圾回收。更好的方法是完全避免使用全局引用，而存储对象引用而不是long。如果执行此操作，则JVM将能够自动收集要分配的所有对象。