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

It accepts a JNIEnv pointer and a class descriptor, and then locates a class loader to load the corresponding class. It returns a local reference to an initialized class, or NULL in case of failure. FindClass uses the class loader associated with the topmost method of the call stack. If it cannot find one, it will use the "system" class loader. One typical example is that after we create a thread and attach it to the VM, the topmost method of the call stack will be as follows:

dalvik.system.NativeStart.run(Native method)

This method is not part of our application code. Therefore the "system" class loader is used.

In our ClassManipulation project, we illustrated FindClass with findClassDemo and findClassDemo2 native methods. The findClassDemo method runs in a VM created thread. The FindClass call will locate the class loader properly. The findClassDemo2 method creates a non-VM thread and attaches the thread to VM. It illustrates the case we described in the preceding section. The logcat output for calling the two native methods is as follows:

As shown in the output, the non-VM thread loads the String class successfully but not the Dummy class defined by us. The way to work around this issue is to cache a reference to the Dummy class in the JNI_OnLoad method. We'll provide a detailed example in the Caching jfieldID, jmethodID, and referencing data to improve performance recipe.

jclass GetSuperclass(JNIEnv *env, jclass clazz);

It helps us to find the superclass of a given class. If clazz is java.lang.Object, this function returns NULL; if it's an interface, it returns a local reference to java.lang.Object; if it's any other class, it returns a local reference to its superclass.

In our ClassManipulation project, we illustrated GetSuperclass with the GetSuperclassDemo native method. We created a Dummy class and a DummyInterface interface in Java code, where DummySubClass extends Dummy, and DummySubInterface extends DummyInterface. In the native method, we then invoked GetSuperclass to java.lang.Object, DummySubClass, and DummySubInterface respectively. The following is a screenshot of the logcat output:

As shown in the screenshot, GetSuperclass can find the superclass of DummySubClass successfully. In this native method, we used a utility function nativeGetClassName, where we called the toString method. We'll cover more about how to make such method calls in the Calling instance and static methods in JNI recipe.

jboolean IsAssignableFrom(JNIEnv *env, jclass cls1, jclass cls2);

This function returns JNI_TRUE if cls1 can be safely casted to cls2, and JNI_FALSE otherwise. We demonstrated its usage with the native method IsAssignableFromDemo. We obtained a local reference to DummySubClass, and called GetSuperclass to get a local reference to Dummy. Then, we called IsAssignableFrom to test if we can cast DummySubClass to Dummy and vice versa. The following is a screenshot of the logcat output:

As expected, the subclass can be safely cast to its superclass, but not the other way round.