1/*
2 * Copyright (C) 2008 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#define LOG_TAG "PlatformLibrary"
18#include "utils/Log.h"
19
20#include <stdlib.h>
21#include <string.h>
22#include <unistd.h>
23#include <assert.h>
24
25#include "jni.h"
26
27
28// ----------------------------------------------------------------------------
29
30/*
31 * Field/method IDs and class object references.
32 *
33 * You should not need to store the JNIEnv pointer in here.  It is
34 * thread-specific and will be passed back in on every call.
35 */
36static struct {
37    jclass      platformLibraryClass;
38    jfieldID    jniInt;
39    jmethodID   yodel;
40} gCachedState;
41
42// ----------------------------------------------------------------------------
43
44/*
45 * Helper function to throw an arbitrary exception.
46 *
47 * Takes the exception class name, a format string, and one optional integer
48 * argument (useful for including an error code, perhaps from errno).
49 */
50static void throwException(JNIEnv* env, const char* ex, const char* fmt,
51    int data) {
52
53    if (jclass cls = env->FindClass(ex)) {
54        if (fmt != NULL) {
55            char msg[1000];
56            snprintf(msg, sizeof(msg), fmt, data);
57            env->ThrowNew(cls, msg);
58        } else {
59            env->ThrowNew(cls, NULL);
60        }
61
62        /*
63         * This is usually not necessary -- local references are released
64         * automatically when the native code returns to the VM.  It's
65         * required if the code doesn't actually return, e.g. it's sitting
66         * in a native event loop.
67         */
68        env->DeleteLocalRef(cls);
69    }
70}
71
72/*
73 * Trivial sample method.
74 *
75 * If "bad" is true, this throws an exception.  Otherwise, this sets the
76 * "mJniInt" field to 42 and returns 24.
77 */
78static jint PlatformLibrary_getJniInt(JNIEnv* env, jobject thiz, jboolean bad) {
79    if (bad) {
80        throwException(env, "java/lang/IllegalStateException",
81                "you are bad", 0);
82        return 0;       /* return value will be ignored */
83    }
84    env->SetIntField(thiz, gCachedState.jniInt, 42);
85    return (jint)24;
86}
87
88/*
89 * A more complex sample method.
90 *
91 * This takes a String as an argument, and returns a new String with
92 * characters in reverse order.  The new string is passed to another method.
93 * This demonstrates basic String manipulation functions and method
94 * invocation.
95 *
96 * This method is declared "static", so there's no "this" pointer; instead,
97 * we get a pointer to the class object.
98 */
99static jstring PlatformLibrary_reverseString(JNIEnv* env, jclass clazz,
100    jstring str) {
101
102    if (str == NULL) {
103        throwException(env, "java/lang/NullPointerException", NULL, 0);
104        return NULL;
105    }
106
107    /*
108     * Get a pointer to the string's UTF-16 character data.  The data
109     * may be a copy or a pointer to the original.  Since String data
110     * is immutable, we're not allowed to touch it.
111     */
112    const jchar* strChars = env->GetStringChars(str, NULL);
113    if (strChars == NULL) {
114        /* something went wrong */
115        ALOGW("Couldn't get string chars\n");
116        return NULL;
117    }
118    jsize strLength = env->GetStringLength(str);
119
120    /*
121     * Write a progress message to the log.  Log messages are UTF-8, so
122     * we want to convert the string to show it.
123     */
124    const char* printable = env->GetStringUTFChars(str, NULL);
125    if (printable != NULL) {
126        ALOGD("Reversing string '%s'\n", printable);
127        env->ReleaseStringUTFChars(str, printable);
128    }
129
130    /*
131     * Copy the characters to temporary storage, reversing as we go.
132     */
133    jchar tempChars[strLength];
134    for (int i = 0; i < strLength; i++) {
135        tempChars[i] = strChars[strLength -1 -i];
136    }
137
138    /*
139     * Release the original String.  That way, if something fails later on,
140     * we don't have to worry about this leading to a memory leak.
141     */
142    env->ReleaseStringChars(str, strChars);
143    strChars = NULL;            /* this pointer no longer valid */
144
145    /*
146     * Create a new String with the chars.
147     */
148    jstring result = env->NewString(tempChars, strLength);
149    if (result == NULL) {
150        ALOGE("NewString failed\n");
151        return NULL;
152    }
153
154    /*
155     * Now let's do something with it.  We already have the methodID for
156     * "yodel", so we can invoke it directly.  It's in our class, so we
157     * can use the Class object reference that was passed in.
158     */
159    env->CallStaticVoidMethod(clazz, gCachedState.yodel, result);
160
161    return result;
162}
163
164
165// ----------------------------------------------------------------------------
166
167/*
168 * Array of methods.
169 *
170 * Each entry has three fields: the name of the method, the method
171 * signature, and a pointer to the native implementation.
172 */
173static const JNINativeMethod gMethods[] = {
174    { "getJniInt",          "(Z)I",
175                        (void*)PlatformLibrary_getJniInt },
176    { "reverseString",      "(Ljava/lang/String;)Ljava/lang/String;",
177                        (void*)PlatformLibrary_reverseString },
178};
179
180/*
181 * Do some (slow-ish) lookups now and save the results.
182 *
183 * Returns 0 on success.
184 */
185static int cacheIds(JNIEnv* env, jclass clazz) {
186    /*
187     * Save the class in case we want to use it later.  Because this is a
188     * reference to the Class object, we need to convert it to a JNI global
189     * reference.
190     */
191    gCachedState.platformLibraryClass = (jclass) env->NewGlobalRef(clazz);
192    if (clazz == NULL) {
193        ALOGE("Can't create new global ref\n");
194        return -1;
195    }
196
197    /*
198     * Cache field and method IDs.  IDs are not references, which means we
199     * don't need to call NewGlobalRef on them.
200     */
201    gCachedState.jniInt = env->GetFieldID(clazz, "mJniInt", "I");
202    if (gCachedState.jniInt == NULL) {
203        ALOGE("Can't find PlatformLibrary.mJniInt\n");
204        return -1;
205    }
206
207    gCachedState.yodel = env->GetStaticMethodID(clazz, "yodel",
208        "(Ljava/lang/String;)V");
209    if (gCachedState.yodel == NULL) {
210        ALOGE("Can't find PlatformLibrary.yodel\n");
211        return -1;
212    }
213
214    return 0;
215}
216
217/*
218 * Explicitly register all methods for our class.
219 *
220 * While we're at it, cache some class references and method/field IDs.
221 *
222 * Returns 0 on success.
223 */
224static int registerMethods(JNIEnv* env) {
225    static const char* const kClassName =
226        "com/example/android/platform_library/PlatformLibrary";
227    jclass clazz;
228
229    /* look up the class */
230    clazz = env->FindClass(kClassName);
231    if (clazz == NULL) {
232        ALOGE("Can't find class %s\n", kClassName);
233        return -1;
234    }
235
236    /* register all the methods */
237    if (env->RegisterNatives(clazz, gMethods,
238            sizeof(gMethods) / sizeof(gMethods[0])) != JNI_OK)
239    {
240        ALOGE("Failed registering methods for %s\n", kClassName);
241        return -1;
242    }
243
244    /* fill out the rest of the ID cache */
245    return cacheIds(env, clazz);
246}
247
248// ----------------------------------------------------------------------------
249
250/*
251 * This is called by the VM when the shared library is first loaded.
252 */
253jint JNI_OnLoad(JavaVM* vm, void* reserved) {
254    JNIEnv* env = NULL;
255    jint result = -1;
256
257    if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
258        ALOGE("ERROR: GetEnv failed\n");
259        goto bail;
260    }
261    assert(env != NULL);
262
263    if (registerMethods(env) != 0) {
264        ALOGE("ERROR: PlatformLibrary native registration failed\n");
265        goto bail;
266    }
267
268    /* success -- return valid version number */
269    result = JNI_VERSION_1_4;
270
271bail:
272    return result;
273}
274