Android JNI – NIO and Reflection Support

NIO Support

The NIO-related entry points allow native code to access java.nio direct buffers. The contents of a direct buffer can, potentially, reside in native memory outside of the ordinary garbage-collected heap. For information about direct buffers, please see New I/O APIs and the specification of the java.nio.ByteBuffer class.

Three new functions introduced in JDK/JRE 1.4 allow JNI code to create, examine, and manipulate direct buffers:

  • NewDirectByteBuffer
  • GetDirectBufferAddress
  • GetDirectBufferCapacity

NewDirectByteBuffer

jobject NewDirectByteBuffer(JNIEnv* env, void* address, jlong capacity);

Allocates and returns a direct java.nio.ByteBuffer referring to the block of memory starting at the memory address address and extending capacity bytes.

Native code that calls this function and returns the resulting byte-buffer object to Java-level code should ensure that the buffer refers to a valid region of memory that is accessible for reading and, if appropriate, writing. An attempt to access an invalid memory location from Java code will either return an arbitrary value, have no visible effect, or cause an unspecified exception to be thrown.

GetDirectBufferAddress

void* GetDirectBufferAddress(JNIEnv* env, jobject buf);

Fetches and returns the starting address of the memory region referenced by the given direct java.nio.Buffer. This function allows native code to access the same memory region that is accessible to Java code via the buffer object.

GetDirectBufferCapacity

jlong GetDirectBufferCapacity(JNIEnv* env, jobject buf);

Fetches and returns the capacity of the memory region referenced by the given direct java.nio.Buffer. The capacity is the number of elements that the memory region contains.

Reflection Support

Programmers can use the JNI to call Java methods or access Java fields if they know the name and type of the methods or fields. The Java Core Reflection API allows programmers to introspect Java classes at runtime. JNI provides a set of conversion functions between field and method IDs used in the JNI to field and method objects used in the Java Core Reflection API.

FromReflectedMethod

jmethodID FromReflectedMethod(JNIEnv *env, jobject method);

Converts a java.lang.reflect.Method or java.lang.reflect.Constructor object to a method ID.

FromReflectedField

jfieldID FromReflectedField(JNIEnv *env, jobject field);

Converts a java.lang.reflect.Field to a field ID.

ToReflectedMethod

jobject ToReflectedMethod(JNIEnv *env, jclass cls, jmethodID methodID, jboolean isStatic);

Converts a method ID derived from cls to a java.lang.reflect.Method or java.lang.reflect.Constructor object. isStatic must be set to JNI_TRUE if the method ID refers to a static field, and JNI_FALSE otherwise.

Throws OutOfMemoryError and returns 0 if fails.

ToReflectedField

jobject ToReflectedField(JNIEnv *env, jclass cls, jfieldID fieldID, jboolean isStatic);

Converts a field ID derived from cls to a java.lang.reflect.Field object. isStatic must be set to JNI_TRUE if fieldID refers to a static field, and JNI_FALSE otherwise.

Throws OutOfMemoryError and returns 0 if fails.

Reference

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

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