Java example source code file (Subclassing.m)

This example Java source code file (Subclassing.m) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

class, jnf_class_cache, jnf_static_member_cache, jnfjobjectwrapper, jnfreleaseenv, jni_false, jnicall, jnienv, jniexport, jobj_ivar_name, nslog, null, sel

The Subclassing.m Java example source code

/*
 * Copyright (c) 2011, 2012, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

#include "com_apple_jobjc_Subclassing.h"

#include <math.h>
#include <assert.h>
#include <errno.h>

#include <objc/runtime.h>
#include <objc/message.h>

#include <ffi/ffi.h>
#include <sys/mman.h>

#include <JavaNativeFoundation/JavaNativeFoundation.h>

// Subclassing of Obj-C classes in Java
//
// See:
//  - Objective-C Runtime documentation
//  - man ffi_prep_closure
//  - Subclassing.java


#pragma mark Accessing object in IVar

#define JOBJ_IVAR_NAME "jObjWrapper"
static jobject getJObjectFromIVar(id obj);

jobject getJObjectFromIVar(id obj)
{
    JNFJObjectWrapper *wrapper = NULL;
    object_getInstanceVariable(obj, JOBJ_IVAR_NAME, (void**) &wrapper);
    return wrapper ? [wrapper jObject] : NULL;
}

JNIEXPORT jobject JNICALL Java_com_apple_jobjc_Subclassing_getJObjectFromIVar
(JNIEnv *env, jclass jClass, jlong jPtr)
{
    id obj = (id) jlong_to_ptr(jPtr);
    if(obj == NULL){
        (*env)->ThrowNew(env, (*env)->FindClass(env,
            "java/lang/NullPointerException"), "obj");
        return NULL;
    }

    JNFJObjectWrapper *wrapper;

    if(!object_getInstanceVariable(obj, JOBJ_IVAR_NAME, (void**) &wrapper)){
        NSLog(@"IVar '%s' not found. obj: %@", JOBJ_IVAR_NAME, obj);
        (*env)->ThrowNew(env, (*env)->FindClass(env, "java/lang/RuntimeException"),
            "Could not find instance variable that holds Java object.");
        return NULL;
    }

    return wrapper ? [wrapper jObject] : NULL;
}

JNIEXPORT void JNICALL Java_com_apple_jobjc_Subclassing_initJObjectToIVar
(JNIEnv *env, jclass jClass, jlong jPtr, jobject jObject)
{
    id obj = (id) jlong_to_ptr(jPtr);
    JNFJObjectWrapper *wrapper = [[JNFJObjectWrapper alloc]
        initWithJObject:jObject withEnv:env];
    [wrapper retain];

    if(!object_setInstanceVariable(obj, JOBJ_IVAR_NAME, wrapper)){
        NSLog(@"IVar '%s' not found. obj: %@", JOBJ_IVAR_NAME, obj);
        (*env)->ThrowNew(env, (*env)->FindClass(env, "java/lang/RuntimeException"),
            "Could not find instance variable that holds Java object.");
        return;
    }
}

#pragma mark Registering class

JNIEXPORT jlong JNICALL Java_com_apple_jobjc_Subclassing_allocateClassPair
(JNIEnv *env, jclass clazz, jlong jSuperClass, jstring jName)
{
    const Class superClass = (Class)jlong_to_ptr(jSuperClass);
    assert(superClass);

    const char *name = (*env)->GetStringUTFChars(env, jName, JNI_FALSE);
    const Class newClass = objc_allocateClassPair(superClass, name, 0);
    (*env)->ReleaseStringUTFChars(env, jName, name);

    return ptr_to_jlong(newClass);
}

JNIEXPORT jboolean JNICALL Java_com_apple_jobjc_Subclassing_addIVarForJObj
(JNIEnv *env, jclass clazz, jlong jSynthClass)
{
    return class_addIvar(
        jlong_to_ptr(jSynthClass),
        JOBJ_IVAR_NAME,
        sizeof(id),
        (uint8_t)log2((double)sizeof(id)),
        "@");
}

JNIEXPORT void JNICALL Java_com_apple_jobjc_Subclassing_registerClassPair
(JNIEnv *env, jclass clazz, jlong jClass)
{
    Class c = jlong_to_ptr(jClass);
//    NSLog(@"Registering class pair %p / %s", c, class_getName(c));
    objc_registerClassPair(c);
}


#pragma mark Patching +alloc

static id patchedAllocIMP(id obj, SEL sel);
static void addJavaInstance(id obj);

JNIEXPORT jboolean JNICALL Java_com_apple_jobjc_Subclassing_patchAlloc
(JNIEnv *env, jclass clazz, jlong jNativeClass)
{
    Class metaClass = object_getClass(jlong_to_ptr(jNativeClass));
    return class_addMethod(metaClass,
       sel_registerName("alloc"),
       (IMP) patchedAllocIMP,
       "@@:");
}

static id patchedAllocIMP(id cls, SEL sel){
    id inst = class_createInstance(cls, 0);
    addJavaInstance(inst);
    return inst;
}

static void addJavaInstance(id obj){
//    NSLog(@"addJavaInstance %p", obj);
//    NSLog(@"... calling up to Java");

    static JNF_CLASS_CACHE(jc_Subclassing, "com/apple/jobjc/Subclassing");
    static JNF_STATIC_MEMBER_CACHE(jm_Subclassing_initJObject,
        jc_Subclassing,
        "initJObject",
        "(J)V");

    JNFThreadContext threadWasAttached = JNFThreadDetachOnThreadDeath;
    JNIEnv *env = JNFObtainEnv(&threadWasAttached);
    JNFCallStaticVoidMethod(env, jm_Subclassing_initJObject,
        ptr_to_jlong(obj));

    JNFReleaseEnv(env, &threadWasAttached);
}


#pragma mark Adding methods

static ffi_closure *make_closure(ffi_cif *cif, void *user_data);
static void sel_closure_call(ffi_cif* cif, void* result, void** args, void* user_data);

typedef struct closure_data_t{
    JNFJObjectWrapper *jMethod;
    JNFJObjectWrapper *jCIF;
} closure_data_t;

static ffi_closure *make_closure(ffi_cif *cif, void *user_data){
    // Allocate a page to hold the closure with read and write permissions.
    ffi_closure *closure;
    if ((closure = mmap(NULL, sizeof(ffi_closure), PROT_READ | PROT_WRITE,
                        MAP_ANON | MAP_PRIVATE, -1, (off_t) 0)) == (void*)-1)
    {
        fprintf(stderr, "mmap failed with errno: %d", errno);
        return NULL;
    }

    // Prepare the ffi_closure structure.
    ffi_status status;
    if ((status = ffi_prep_closure(closure, cif, sel_closure_call, (void *)user_data)) != FFI_OK)
    {
        fprintf(stderr, "ffi_prep_closure failed with ffi_status: %d", status);
        munmap(closure, sizeof(ffi_closure));
        return NULL;
    }

    // Ensure that the closure will execute on all architectures.
    if (mprotect(closure, sizeof(closure), PROT_READ | PROT_EXEC) == -1)
    {
        fprintf(stderr, "mprotect failed with errno: %d", errno);
        munmap(closure, sizeof(ffi_closure));
        return NULL;
    }
    return closure;
}

JNIEXPORT jboolean JNICALL Java_com_apple_jobjc_Subclassing_addMethod
(JNIEnv *env, jclass clazz, jlong jClass, jstring jSelName, jobject jMethod,
    jobject jCIF, jlong jCIFPtr, jstring jObjCEncodedType)
{
    ffi_cif *cif = jlong_to_ptr(jCIFPtr);

    closure_data_t *user_data = malloc(sizeof(closure_data_t));
    user_data->jMethod = [[JNFJObjectWrapper alloc] initWithJObject:jMethod withEnv:env];
    user_data->jCIF = [[JNFJObjectWrapper alloc] initWithJObject:jCIF withEnv:env];

    ffi_closure *closure;;
    if(!(closure = make_closure(cif, user_data))){
        [user_data->jMethod release];
        [user_data->jCIF release];
        free(user_data);
        return NO;
    }

    const Class objcClass = (Class)jlong_to_ptr(jClass);

    const char *selName = (*env)->GetStringUTFChars(env, jSelName, JNI_FALSE);
    const char *objCEncodedType = (*env)->GetStringUTFChars(env, jObjCEncodedType, JNI_FALSE);

//    NSLog(@"Adding method '%s' :: '%s' to '%s' / %p",
//        selName,
//        objCEncodedType,
//        class_getName(objcClass),
//        objcClass);

    BOOL ret = class_addMethod(objcClass, sel_registerName(selName), (IMP) closure, objCEncodedType);

    (*env)->ReleaseStringUTFChars(env, jSelName, selName);
    (*env)->ReleaseStringUTFChars(env, jObjCEncodedType, objCEncodedType);

    if(!ret){
        NSLog(@"class_addMethod failed");
        munmap(closure, sizeof(ffi_closure));
        [user_data->jMethod release];
        [user_data->jCIF release];
        free(user_data);
        return NO;
    }

    return ret;
}

static void sel_closure_call(ffi_cif* cif, void* result, void** args, void* user_data)
{
    id obj = *(id*) args[0];
//    SEL sel = *(SEL*) args[1];

//    NSLog(@"Subclassing: sel_closure_call: %p %p", obj, sel);
//    NSLog(@"Subclassing: sel_closure_call: obj class: %@  sel name: %s", object_getClass(obj), sel_getName(sel));

    jobject jObj = getJObjectFromIVar(obj);

    if(!jObj){
        addJavaInstance(obj);
        jObj = getJObjectFromIVar(obj);
    }

    closure_data_t *jmeta = user_data;
    jobject jMethod = [jmeta->jMethod jObject];
    jobject jCIF = [jmeta->jCIF jObject];

    JNFThreadContext threadWasAttached = JNFThreadDetachOnThreadDeath;
    JNIEnv *env = JNFObtainEnv(&threadWasAttached);

    if((*env)->ExceptionOccurred(env)) goto bail;

        static JNF_CLASS_CACHE(jc, "com/apple/jobjc/Subclassing");
        static JNF_STATIC_MEMBER_CACHE(jm_invokeFromJNI, jc, "invokeFromJNI",
        "(Lcom/apple/jobjc/ID;Ljava/lang/reflect/Method;Lcom/apple/jobjc/CIF;JJ)V");

    JNFCallStaticVoidMethod(env, jm_invokeFromJNI,
        jObj,
        jMethod,
        jCIF,
        ptr_to_jlong(result),
        ptr_to_jlong(args));

bail:
    JNFReleaseEnv(env, &threadWasAttached);

    if((*env)->ExceptionOccurred(env)){
        NSLog(@"Exception!");
        (*env)->ExceptionDescribe(env);
    }
    JNFReleaseEnv(env, &threadWasAttached);
}