Android JNI – Class Operations, Exceptions and References

Class Operations

DefineClass

Android does not use Java bytecodes or class files, so passing in binary class data doesn’t work.

FindClass

jclass FindClass(JNIEnv *env, const char *name);

The name argument is a fully-qualified class name or an array type signature.

For example, the fully-qualified class name for the java.lang.String class is: “java/lang/String”

Exceptions

Throw

jint Throw(JNIEnv *env, jthrowable obj);

Causes a java.lang.Throwable object to be thrown.

ThrowNew

jint ThrowNew(JNIEnv *env, jclass clazz, const char *message);

Constructs an exception object from the specified class with the message specified by message and causes that exception to be thrown.

ExceptionOccurred

jthrowable ExceptionOccurred(JNIEnv *env);

Determines if an exception is being thrown. The exception stays being thrown until either the native code calls ExceptionClear(), or the Java code handles the exception.

ExceptionDescribe

void ExceptionDescribe(JNIEnv *env);

Prints an exception and a backtrace of the stack to a system error-reporting channel, such as stderr. This is a convenience routine provided for debugging.

ExceptionClear

void ExceptionClear(JNIEnv *env);

Clears any exception that is currently being thrown. If no exception is currently being thrown, this routine has no effect.

FatalError

void FatalError(JNIEnv *env, const char *msg);

Raises a fatal error and does not expect the VM to recover. This function does not return.

ExceptionCheck

We introduce a convenience function to check for pending exceptions without creating a local reference to the exception object.

jboolean ExceptionCheck(JNIEnv *env);

Returns JNI_TRUE when there is a pending exception; otherwise, returns JNI_FALSE.

Global References

NewGlobalRef

jobject NewGlobalRef(JNIEnv *env, jobject obj);

Creates a new global reference to the object referred to by the obj argument. The obj argument may be a global or local reference. Global references must be explicitly dsposed of by calling DeleteGlobalRef().

DeleteGlobalRef

void DeleteGlobalRef(JNIEnv *env, jobject globalRef);

Deletes the global reference pointed to by globalRef.

Local References

Local references are valid for the duration of a native method call. They are freed automatically after the native method returns. Each local reference costs some amount of Java Virtual Machine resource. Programmers need to make sure that native methods do not excessively allocate local references. Although local references are automatically freed after the native method returns to Java, excessive allocation of local references may cause the VM to run out of memory during the execution of a native method.

DeleteLocalRef

void DeleteLocalRef(JNIEnv *env, jobject localRef);

NewLocalRef

jobject NewLocalRef(JNIEnv *env, jobject ref);

Creates a new local reference that refers to the same object as ref. The given ref may be a global or local reference. Returns NULL if ref refers to null.

Weak Global References

Weak global references are a special kind of global reference. Unlike normal global references, a weak global reference allows the underlying Java object to be garbage collected. Weak global references may be used in any situation where global or local references are used. When the garbage collector runs, it frees the underlying object if the object is only referred to by weak references. A weak global reference pointing to a freed object is functionally equivalent to NULL. Programmers can detect whether a weak global reference points to a freed object by using IsSameObject to compare the weak reference against NULL.

NewWeakGlobalRef

jweak NewWeakGlobalRef(JNIEnv *env, jobject obj);

Creates a new weak global reference. Returns NULL if obj refers to null, or if the VM runs out of memory. If the VM runs out of memory, an OutOfMemoryError will be thrown.

DeleteWeakGlobalRef

void DeleteWeakGlobalRef(JNIEnv *env, jweak obj);

Delete the VM resources needed for the given weak global reference.

Reference

https://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/jniTOC.html 

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