/*
* Copyright (C) 2008 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#define LOG_TAG "PlatformLibrary"
#include "utils/Log.h"
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <assert.h>
#include "jni.h"
// ----------------------------------------------------------------------------
/*
* Field/method IDs and class object references.
*
* You should not need to store the JNIEnv pointer in here. It is
* thread-specific and will be passed back in on every call.
*/
static struct {
jclass platformLibraryClass;
jfieldID jniInt;
jmethodID yodel;
} gCachedState;
// ----------------------------------------------------------------------------
/*
* Helper function to throw an arbitrary exception.
*
* Takes the exception class name, a format string, and one optional integer
* argument (useful for including an error code, perhaps from errno).
*/
static void throwException(JNIEnv* env, const char* ex, const char* fmt,
int data) {
if (jclass cls = env->FindClass(ex)) {
if (fmt != NULL) {
char msg[1000];
snprintf(msg, sizeof(msg), fmt, data);
env->ThrowNew(cls, msg);
} else {
env->ThrowNew(cls, NULL);
}
/*
* This is usually not necessary -- local references are released
* automatically when the native code returns to the VM. It's
* required if the code doesn't actually return, e.g. it's sitting
* in a native event loop.
*/
env->DeleteLocalRef(cls);
}
}
/*
* Trivial sample method.
*
* If "bad" is true, this throws an exception. Otherwise, this sets the
* "mJniInt" field to 42 and returns 24.
*/
static jint PlatformLibrary_getJniInt(JNIEnv* env, jobject thiz, jboolean bad) {
if (bad) {
throwException(env, "java/lang/IllegalStateException",
"you are bad", 0);
return 0; /* return value will be ignored */
}
env->SetIntField(thiz, gCachedState.jniInt, 42);
return (jint)24;
}
/*
* A more complex sample method.
*
* This takes a String as an argument, and returns a new String with
* characters in reverse order. The new string is passed to another method.
* This demonstrates basic String manipulation functions and method
* invocation.
*
* This method is declared "static", so there's no "this" pointer; instead,
* we get a pointer to the class object.
*/
static jstring PlatformLibrary_reverseString(JNIEnv* env, jclass clazz,
jstring str) {
if (str == NULL) {
throwException(env, "java/lang/NullPointerException", NULL, 0);
return NULL;
}
/*
* Get a pointer to the string's UTF-16 character data. The data
* may be a copy or a pointer to the original. Since String data
* is immutable, we're not allowed to touch it.
*/
const jchar* strChars = env->GetStringChars(str, NULL);
if (strChars == NULL) {
/* something went wrong */
ALOGW("Couldn't get string chars\n");
return NULL;
}
jsize strLength = env->GetStringLength(str);
/*
* Write a progress message to the log. Log messages are UTF-8, so
* we want to convert the string to show it.
*/
const char* printable = env->GetStringUTFChars(str, NULL);
if (printable != NULL) {
ALOGD("Reversing string '%s'\n", printable);
env->ReleaseStringUTFChars(str, printable);
}
/*
* Copy the characters to temporary storage, reversing as we go.
*/
jchar tempChars[strLength];
for (int i = 0; i < strLength; i++) {
tempChars[i] = strChars[strLength -1 -i];
}
/*
* Release the original String. That way, if something fails later on,
* we don't have to worry about this leading to a memory leak.
*/
env->ReleaseStringChars(str, strChars);
strChars = NULL; /* this pointer no longer valid */
/*
* Create a new String with the chars.
*/
jstring result = env->NewString(tempChars, strLength);
if (result == NULL) {
ALOGE("NewString failed\n");
return NULL;
}
/*
* Now let's do something with it. We already have the methodID for
* "yodel", so we can invoke it directly. It's in our class, so we
* can use the Class object reference that was passed in.
*/
env->CallStaticVoidMethod(clazz, gCachedState.yodel, result);
return result;
}
// ----------------------------------------------------------------------------
/*
* Array of methods.
*
* Each entry has three fields: the name of the method, the method
* signature, and a pointer to the native implementation.
*/
static const JNINativeMethod gMethods[] = {
{ "getJniInt", "(Z)I",
(void*)PlatformLibrary_getJniInt },
{ "reverseString", "(Ljava/lang/String;)Ljava/lang/String;",
(void*)PlatformLibrary_reverseString },
};
/*
* Do some (slow-ish) lookups now and save the results.
*
* Returns 0 on success.
*/
static int cacheIds(JNIEnv* env, jclass clazz) {
/*
* Save the class in case we want to use it later. Because this is a
* reference to the Class object, we need to convert it to a JNI global
* reference.
*/
gCachedState.platformLibraryClass = (jclass) env->NewGlobalRef(clazz);
if (clazz == NULL) {
ALOGE("Can't create new global ref\n");
return -1;
}
/*
* Cache field and method IDs. IDs are not references, which means we
* don't need to call NewGlobalRef on them.
*/
gCachedState.jniInt = env->GetFieldID(clazz, "mJniInt", "I");
if (gCachedState.jniInt == NULL) {
ALOGE("Can't find PlatformLibrary.mJniInt\n");
return -1;
}
gCachedState.yodel = env->GetStaticMethodID(clazz, "yodel",
"(Ljava/lang/String;)V");
if (gCachedState.yodel == NULL) {
ALOGE("Can't find PlatformLibrary.yodel\n");
return -1;
}
return 0;
}
/*
* Explicitly register all methods for our class.
*
* While we're at it, cache some class references and method/field IDs.
*
* Returns 0 on success.
*/
static int registerMethods(JNIEnv* env) {
static const char* const kClassName =
"com/example/android/platform_library/PlatformLibrary";
jclass clazz;
/* look up the class */
clazz = env->FindClass(kClassName);
if (clazz == NULL) {
ALOGE("Can't find class %s\n", kClassName);
return -1;
}
/* register all the methods */
if (env->RegisterNatives(clazz, gMethods,
sizeof(gMethods) / sizeof(gMethods[0])) != JNI_OK)
{
ALOGE("Failed registering methods for %s\n", kClassName);
return -1;
}
/* fill out the rest of the ID cache */
return cacheIds(env, clazz);
}
// ----------------------------------------------------------------------------
/*
* This is called by the VM when the shared library is first loaded.
*/
jint JNI_OnLoad(JavaVM* vm, void* reserved) {
JNIEnv* env = NULL;
jint result = -1;
if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
ALOGE("ERROR: GetEnv failed\n");
goto bail;
}
assert(env != NULL);
if (registerMethods(env) != 0) {
ALOGE("ERROR: PlatformLibrary native registration failed\n");
goto bail;
}
/* success -- return valid version number */
result = JNI_VERSION_1_4;
bail:
return result;
}