Android中关于JNI 的学习（一）对于JNIEnv的一些认识
Java 通过 JNI 机制调用 c/c++ 写的 native 程序。c/c++ 开发的 native 程序需要遵循一定的 JNI 规范，下面的例子就是一个 JNI 函数声明：
1 JNIEXPORT jstring JNICALL Java_com_clay_example_JNITest_getJNIString 2 (JNIEnv* env, jobject obj)
JVM 负责从Java Stack 转入 C/C++ Native Stack。当 Java 进入 JNI 调用，除了函数本身的参数(arg0),会多出两个参数：JNIEnv 指针和 jobject 指针。而我们在Java端定义这个方法的时候，是没有参数的，如下：
1 public native String printHello();
JNIEnv 指针是 JVM 创建的，用于 Native的 c/c++ 方法操纵 Java 执行栈中的数据，比如 Java Class, Java Method 等。
首先，JNI 对于JNIEnv 的使用, 提供了两种语法: c 语法以及 c++ 语法，如下：
c 语法：
1 jsize len = (*env)->GetArrayLength(env,array);
c++ 语法：
1 jsize len =env->GetArrayLength(array);
那么这个 JNIEnv 是干什么用的？ 其实从这个参数的名称就可以看到，就是指 JNI 的运行环境，我觉得它就是对 Java 虚拟环境的一个引用，在 Android 中，就是指 Dalvik VM。 参考 jni.h 文件中关于 JNIEnv 的定义，如下（对于 C 和 C++，它的定义有点不一样）：
1 struct _JNIEnv; 2 struct _JavaVM; 3 typedef const struct JNINativeInterface* C_JNIEnv; 4 5 #if defined(__cplusplus) 6 typedef _JNIEnv JNIEnv; 7 typedef _JavaVM JavaVM; 8 #else 9 typedef const struct JNINativeInterface* JNIEnv; 10 typedef const struct JNIInvokeInterface* JavaVM; 11 #endif
在 C 中，我们可以看到 JNIEnv 的类型就是 JNINativeInterface* ，是一个指针类型，那么在 C++中呢，_JNIEnv 是什么样的呢？
1 /*
2 * C++ object wrapper.
3 *
4 * This is usually overlaid on a C struct whose first element is a
5 * JNINativeInterface*. We rely somewhat on compiler behavior.
6 */
7 struct _JNIEnv {
8 /* do not rename this; it does not seem to be entirely opaque */
9 const struct JNINativeInterface* functions;
10
11 #if defined(__cplusplus)
12
13 jint GetVersion()
14 { return functions->GetVersion(this); }
而对于 C++ 来说， _JNIEnv 是一个结构体，里面包含了 JNINativeInterface* 的结构。
所以从这里也可以看到，对于 C 和 C++ 来说，它们引用 JNIEnv 中的方法是有一点不一样的。 总的来说，JNIEnv，不管是 C，还是 C++，其实关键都是 JNINativeInterface 的这个结构。
我们可以简单看一下 JNINativeInterface 结构的定义，如下：
1 /*
2 * Table of interface function pointers.
3 */
4 struct JNINativeInterface {
5 void* reserved0;
6 void* reserved1;
7 void* reserved2;
8 void* reserved3;
9
10 jint (*GetVersion)(JNIEnv *);
11
12 jclass (*DefineClass)(JNIEnv*, const char*, jobject, const jbyte*,
13 jsize);
14 jclass (*FindClass)(JNIEnv*, const char*);
15
16 jmethodID (*FromReflectedMethod)(JNIEnv*, jobject);
17 jfieldID (*FromReflectedField)(JNIEnv*, jobject);
18 /* spec doesn't show jboolean parameter */
19 jobject (*ToReflectedMethod)(JNIEnv*, jclass, jmethodID, jboolean);
20
21 jclass (*GetSuperclass)(JNIEnv*, jclass);
22 jboolean (*IsAssignableFrom)(JNIEnv*, jclass, jclass);
23
24 /* spec doesn't show jboolean parameter */
25 jobject (*ToReflectedField)(JNIEnv*, jclass, jfieldID, jboolean);
26
27 jint (*Throw)(JNIEnv*, jthrowable);
28 jint (*ThrowNew)(JNIEnv *, jclass, const char *);
29 jthrowable (*ExceptionOccurred)(JNIEnv*);
30 void (*ExceptionDescribe)(JNIEnv*);
31 void (*ExceptionClear)(JNIEnv*);
32 void (*FatalError)(JNIEnv*, const char*);
33
34 jint (*PushLocalFrame)(JNIEnv*, jint);
35 jobject (*PopLocalFrame)(JNIEnv*, jobject);
36
37 jobject (*NewGlobalRef)(JNIEnv*, jobject);
38 void (*DeleteGlobalRef)(JNIEnv*, jobject);
39 void (*DeleteLocalRef)(JNIEnv*, jobject);
40 jboolean (*IsSameObject)(JNIEnv*, jobject, jobject);
41
42 jobject (*NewLocalRef)(JNIEnv*, jobject);
43 jint (*EnsureLocalCapacity)(JNIEnv*, jint);
44
45 jobject (*AllocObject)(JNIEnv*, jclass);
46 jobject (*NewObject)(JNIEnv*, jclass, jmethodID, ...);
47 jobject (*NewObjectV)(JNIEnv*, jclass, jmethodID, va_list);
48 jobject (*NewObjectA)(JNIEnv*, jclass, jmethodID, const jvalue*);
49
50 jclass (*GetObjectClass)(JNIEnv*, jobject);
51 jboolean (*IsInstanceOf)(JNIEnv*, jobject, jclass);
52 jmethodID (*GetMethodID)(JNIEnv*, jclass, const char*, const char*);
53
54 jobject (*CallObjectMethod)(JNIEnv*, jobject, jmethodID, ...);
55 jobject (*CallObjectMethodV)(JNIEnv*, jobject, jmethodID, va_list);
56 jobject (*CallObjectMethodA)(JNIEnv*, jobject, jmethodID, const jvalue*);
57 jboolean (*CallBooleanMethod)(JNIEnv*, jobject, jmethodID, ...);
58 jboolean (*CallBooleanMethodV)(JNIEnv*, jobject, jmethodID, va_list);
59 jboolean (*CallBooleanMethodA)(JNIEnv*, jobject, jmethodID, const jvalue*);
60 jbyte (*CallByteMethod)(JNIEnv*, jobject, jmethodID, ...);
61 jbyte (*CallByteMethodV)(JNIEnv*, jobject, jmethodID, va_list);
62 jbyte (*CallByteMethodA)(JNIEnv*, jobject, jmethodID, const jvalue*);
63 jchar (*CallCharMethod)(JNIEnv*, jobject, jmethodID, ...);
64 jchar (*CallCharMethodV)(JNIEnv*, jobject, jmethodID, va_list);
65 jchar (*CallCharMethodA)(JNIEnv*, jobject, jmethodID, const jvalue*);
66 jshort (*CallShortMethod)(JNIEnv*, jobject, jmethodID, ...);
67 jshort (*CallShortMethodV)(JNIEnv*, jobject, jmethodID, va_list);
68 jshort (*CallShortMethodA)(JNIEnv*, jobject, jmethodID, const jvalue*);
69 jint (*CallIntMethod)(JNIEnv*, jobject, jmethodID, ...);
70 jint (*CallIntMethodV)(JNIEnv*, jobject, jmethodID, va_list);
71 jint (*CallIntMethodA)(JNIEnv*, jobject, jmethodID, const jvalue*);
72 jlong (*CallLongMethod)(JNIEnv*, jobject, jmethodID, ...);
73 jlong (*CallLongMethodV)(JNIEnv*, jobject, jmethodID, va_list);
74 jlong (*CallLongMethodA)(JNIEnv*, jobject, jmethodID, const jvalue*);
75 jfloat (*CallFloatMethod)(JNIEnv*, jobject, jmethodID, ...);
76 jfloat (*CallFloatMethodV)(JNIEnv*, jobject, jmethodID, va_list);
77 jfloat (*CallFloatMethodA)(JNIEnv*, jobject, jmethodID, const jvalue*);
78 jdouble (*CallDoubleMethod)(JNIEnv*, jobject, jmethodID, ...);
79 jdouble (*CallDoubleMethodV)(JNIEnv*, jobject, jmethodID, va_list);
80 jdouble (*CallDoubleMethodA)(JNIEnv*, jobject, jmethodID, const jvalue*);
81 void (*CallVoidMethod)(JNIEnv*, jobject, jmethodID, ...);
82 void (*CallVoidMethodV)(JNIEnv*, jobject, jmethodID, va_list);
83 void (*CallVoidMethodA)(JNIEnv*, jobject, jmethodID, const jvalue*);
84
85 jobject (*CallNonvirtualObjectMethod)(JNIEnv*, jobject, jclass,
86 jmethodID, ...);
87 jobject (*CallNonvirtualObjectMethodV)(JNIEnv*, jobject, jclass,
88 jmethodID, va_list);
89 jobject (*CallNonvirtualObjectMethodA)(JNIEnv*, jobject, jclass,
90 jmethodID, const jvalue*);
91 jboolean (*CallNonvirtualBooleanMethod)(JNIEnv*, jobject, jclass,
92 jmethodID, ...);
93 jboolean (*CallNonvirtualBooleanMethodV)(JNIEnv*, jobject, jclass,
94 jmethodID, va_list);
95 jboolean (*CallNonvirtualBooleanMethodA)(JNIEnv*, jobject, jclass,
96 jmethodID, const jvalue*);
97 jbyte (*CallNonvirtualByteMethod)(JNIEnv*, jobject, jclass,
98 jmethodID, ...);
99 jbyte (*CallNonvirtualByteMethodV)(JNIEnv*, jobject, jclass,
100 jmethodID, va_list);
101 jbyte (*CallNonvirtualByteMethodA)(JNIEnv*, jobject, jclass,
102 jmethodID, const jvalue*);
103 jchar (*CallNonvirtualCharMethod)(JNIEnv*, jobject, jclass,
104 jmethodID, ...);
105 jchar (*CallNonvirtualCharMethodV)(JNIEnv*, jobject, jclass,
106 jmethodID, va_list);
107 jchar (*CallNonvirtualCharMethodA)(JNIEnv*, jobject, jclass,
108 jmethodID, const jvalue*);
109 jshort (*CallNonvirtualShortMethod)(JNIEnv*, jobject, jclass,
110 jmethodID, ...);
111 jshort (*CallNonvirtualShortMethodV)(JNIEnv*, jobject, jclass,
112 jmethodID, va_list);
113 jshort (*CallNonvirtualShortMethodA)(JNIEnv*, jobject, jclass,
114 jmethodID, const jvalue*);
115 jint (*CallNonvirtualIntMethod)(JNIEnv*, jobject, jclass,
116 jmethodID, ...);
117 jint (*CallNonvirtualIntMethodV)(JNIEnv*, jobject, jclass,
118 jmethodID, va_list);
119 jint (*CallNonvirtualIntMethodA)(JNIEnv*, jobject, jclass,
120 jmethodID, const jvalue*);
121 jlong (*CallNonvirtualLongMethod)(JNIEnv*, jobject, jclass,
122 jmethodID, ...);
123 jlong (*CallNonvirtualLongMethodV)(JNIEnv*, jobject, jclass,
124 jmethodID, va_list);
125 jlong (*CallNonvirtualLongMethodA)(JNIEnv*, jobject, jclass,
126 jmethodID, const jvalue*);
127 jfloat (*CallNonvirtualFloatMethod)(JNIEnv*, jobject, jclass,
128 jmethodID, ...);
129 jfloat (*CallNonvirtualFloatMethodV)(JNIEnv*, jobject, jclass,
130 jmethodID, va_list);
131 jfloat (*CallNonvirtualFloatMethodA)(JNIEnv*, jobject, jclass,
132 jmethodID, const jvalue*);
133 jdouble (*CallNonvirtualDoubleMethod)(JNIEnv*, jobject, jclass,
134 jmethodID, ...);
135 jdouble (*CallNonvirtualDoubleMethodV)(JNIEnv*, jobject, jclass,
136 jmethodID, va_list);
137 jdouble (*CallNonvirtualDoubleMethodA)(JNIEnv*, jobject, jclass,
138 jmethodID, const jvalue*);
139 void (*CallNonvirtualVoidMethod)(JNIEnv*, jobject, jclass,
140 jmethodID, ...);
141 void (*CallNonvirtualVoidMethodV)(JNIEnv*, jobject, jclass,
142 jmethodID, va_list);
143 void (*CallNonvirtualVoidMethodA)(JNIEnv*, jobject, jclass,
144 jmethodID, const jvalue*);
145
146 jfieldID (*GetFieldID)(JNIEnv*, jclass, const char*, const char*);
147
148 jobject (*GetObjectField)(JNIEnv*, jobject, jfieldID);
149 jboolean (*GetBooleanField)(JNIEnv*, jobject, jfieldID);
150 jbyte (*GetByteField)(JNIEnv*, jobject, jfieldID);
151 jchar (*GetCharField)(JNIEnv*, jobject, jfieldID);
152 jshort (*GetShortField)(JNIEnv*, jobject, jfieldID);
153 jint (*GetIntField)(JNIEnv*, jobject, jfieldID);
154 jlong (*GetLongField)(JNIEnv*, jobject, jfieldID);
155 jfloat (*GetFloatField)(JNIEnv*, jobject, jfieldID);
156 jdouble (*GetDoubleField)(JNIEnv*, jobject, jfieldID);
157
158 void (*SetObjectField)(JNIEnv*, jobject, jfieldID, jobject);
159 void (*SetBooleanField)(JNIEnv*, jobject, jfieldID, jboolean);
160 void (*SetByteField)(JNIEnv*, jobject, jfieldID, jbyte);
161 void (*SetCharField)(JNIEnv*, jobject, jfieldID, jchar);
162 void (*SetShortField)(JNIEnv*, jobject, jfieldID, jshort);
163 void (*SetIntField)(JNIEnv*, jobject, jfieldID, jint);
164 void (*SetLongField)(JNIEnv*, jobject, jfieldID, jlong);
165 void (*SetFloatField)(JNIEnv*, jobject, jfieldID, jfloat);
166 void (*SetDoubleField)(JNIEnv*, jobject, jfieldID, jdouble);
167
168 jmethodID (*GetStaticMethodID)(JNIEnv*, jclass, const char*, const char*);
169
170 jobject (*CallStaticObjectMethod)(JNIEnv*, jclass, jmethodID, ...);
171 jobject (*CallStaticObjectMethodV)(JNIEnv*, jclass, jmethodID, va_list);
172 jobject (*CallStaticObjectMethodA)(JNIEnv*, jclass, jmethodID, const jvalue*);
173 jboolean (*CallStaticBooleanMethod)(JNIEnv*, jclass, jmethodID, ...);
174 jboolean (*CallStaticBooleanMethodV)(JNIEnv*, jclass, jmethodID,
175 va_list);
176 jboolean (*CallStaticBooleanMethodA)(JNIEnv*, jclass, jmethodID, const jvalue*);
177 jbyte (*CallStaticByteMethod)(JNIEnv*, jclass, jmethodID, ...);
178 jbyte (*CallStaticByteMethodV)(JNIEnv*, jclass, jmethodID, va_list);
179 jbyte (*CallStaticByteMethodA)(JNIEnv*, jclass, jmethodID, const jvalue*);
180 jchar (*CallStaticCharMethod)(JNIEnv*, jclass, jmethodID, ...);
181 jchar (*CallStaticCharMethodV)(JNIEnv*, jclass, jmethodID, va_list);
182 jchar (*CallStaticCharMethodA)(JNIEnv*, jclass, jmethodID, const jvalue*);
183 jshort (*CallStaticShortMethod)(JNIEnv*, jclass, jmethodID, ...);
184 jshort (*CallStaticShortMethodV)(JNIEnv*, jclass, jmethodID, va_list);
185 jshort (*CallStaticShortMethodA)(JNIEnv*, jclass, jmethodID, const jvalue*);
186 jint (*CallStaticIntMethod)(JNIEnv*, jclass, jmethodID, ...);
187 jint (*CallStaticIntMethodV)(JNIEnv*, jclass, jmethodID, va_list);
188 jint (*CallStaticIntMethodA)(JNIEnv*, jclass, jmethodID, const jvalue*);
189 jlong (*CallStaticLongMethod)(JNIEnv*, jclass, jmethodID, ...);
190 jlong (*CallStaticLongMethodV)(JNIEnv*, jclass, jmethodID, va_list);
191 jlong (*CallStaticLongMethodA)(JNIEnv*, jclass, jmethodID, const jvalue*);
192 jfloat (*CallStaticFloatMethod)(JNIEnv*, jclass, jmethodID, ...);
193 jfloat (*CallStaticFloatMethodV)(JNIEnv*, jclass, jmethodID, va_list);
194 jfloat (*CallStaticFloatMethodA)(JNIEnv*, jclass, jmethodID, const jvalue*);
195 jdouble (*CallStaticDoubleMethod)(JNIEnv*, jclass, jmethodID, ...);
196 jdouble (*CallStaticDoubleMethodV)(JNIEnv*, jclass, jmethodID, va_list);
197 jdouble (*CallStaticDoubleMethodA)(JNIEnv*, jclass, jmethodID, const jvalue*);
198 void (*CallStaticVoidMethod)(JNIEnv*, jclass, jmethodID, ...);
199 void (*CallStaticVoidMethodV)(JNIEnv*, jclass, jmethodID, va_list);
200 void (*CallStaticVoidMethodA)(JNIEnv*, jclass, jmethodID, const jvalue*);
201
202 jfieldID (*GetStaticFieldID)(JNIEnv*, jclass, const char*,
203 const char*);
204
205 jobject (*GetStaticObjectField)(JNIEnv*, jclass, jfieldID);
206 jboolean (*GetStaticBooleanField)(JNIEnv*, jclass, jfieldID);
207 jbyte (*GetStaticByteField)(JNIEnv*, jclass, jfieldID);
208 jchar (*GetStaticCharField)(JNIEnv*, jclass, jfieldID);
209 jshort (*GetStaticShortField)(JNIEnv*, jclass, jfieldID);
210 jint (*GetStaticIntField)(JNIEnv*, jclass, jfieldID);
211 jlong (*GetStaticLongField)(JNIEnv*, jclass, jfieldID);
212 jfloat (*GetStaticFloatField)(JNIEnv*, jclass, jfieldID);
213 jdouble (*GetStaticDoubleField)(JNIEnv*, jclass, jfieldID);
214
215 void (*SetStaticObjectField)(JNIEnv*, jclass, jfieldID, jobject);
216 void (*SetStaticBooleanField)(JNIEnv*, jclass, jfieldID, jboolean);
217 void (*SetStaticByteField)(JNIEnv*, jclass, jfieldID, jbyte);
218 void (*SetStaticCharField)(JNIEnv*, jclass, jfieldID, jchar);
219 void (*SetStaticShortField)(JNIEnv*, jclass, jfieldID, jshort);
220 void (*SetStaticIntField)(JNIEnv*, jclass, jfieldID, jint);
221 void (*SetStaticLongField)(JNIEnv*, jclass, jfieldID, jlong);
222 void (*SetStaticFloatField)(JNIEnv*, jclass, jfieldID, jfloat);
223 void (*SetStaticDoubleField)(JNIEnv*, jclass, jfieldID, jdouble);
224
225 jstring (*NewString)(JNIEnv*, const jchar*, jsize);
226 jsize (*GetStringLength)(JNIEnv*, jstring);
227 const jchar* (*GetStringChars)(JNIEnv*, jstring, jboolean*);
228 void (*ReleaseStringChars)(JNIEnv*, jstring, const jchar*);
229 jstring (*NewStringUTF)(JNIEnv*, const char*);
230 jsize (*GetStringUTFLength)(JNIEnv*, jstring);
231 /* JNI spec says this returns const jbyte*, but that's inconsistent */
232 const char* (*GetStringUTFChars)(JNIEnv*, jstring, jboolean*);
233 void (*ReleaseStringUTFChars)(JNIEnv*, jstring, const char*);
234 jsize (*GetArrayLength)(JNIEnv*, jarray);
235 jobjectArray (*NewObjectArray)(JNIEnv*, jsize, jclass, jobject);
236 jobject (*GetObjectArrayElement)(JNIEnv*, jobjectArray, jsize);
237 void (*SetObjectArrayElement)(JNIEnv*, jobjectArray, jsize, jobject);
238
239 jbooleanArray (*NewBooleanArray)(JNIEnv*, jsize);
240 jbyteArray (*NewByteArray)(JNIEnv*, jsize);
241 jcharArray (*NewCharArray)(JNIEnv*, jsize);
242 jshortArray (*NewShortArray)(JNIEnv*, jsize);
243 jintArray (*NewIntArray)(JNIEnv*, jsize);
244 jlongArray (*NewLongArray)(JNIEnv*, jsize);
245 jfloatArray (*NewFloatArray)(JNIEnv*, jsize);
246 jdoubleArray (*NewDoubleArray)(JNIEnv*, jsize);
247
248 jboolean* (*GetBooleanArrayElements)(JNIEnv*, jbooleanArray, jboolean*);
249 jbyte* (*GetByteArrayElements)(JNIEnv*, jbyteArray, jboolean*);
250 jchar* (*GetCharArrayElements)(JNIEnv*, jcharArray, jboolean*);
251 jshort* (*GetShortArrayElements)(JNIEnv*, jshortArray, jboolean*);
252 jint* (*GetIntArrayElements)(JNIEnv*, jintArray, jboolean*);
253 jlong* (*GetLongArrayElements)(JNIEnv*, jlongArray, jboolean*);
254 jfloat* (*GetFloatArrayElements)(JNIEnv*, jfloatArray, jboolean*);
255 jdouble* (*GetDoubleArrayElements)(JNIEnv*, jdoubleArray, jboolean*);
256
257 void (*ReleaseBooleanArrayElements)(JNIEnv*, jbooleanArray,
258 jboolean*, jint);
259 void (*ReleaseByteArrayElements)(JNIEnv*, jbyteArray,
260 jbyte*, jint);
261 void (*ReleaseCharArrayElements)(JNIEnv*, jcharArray,
262 jchar*, jint);
263 void (*ReleaseShortArrayElements)(JNIEnv*, jshortArray,
264 jshort*, jint);
265 void (*ReleaseIntArrayElements)(JNIEnv*, jintArray,
266 jint*, jint);
267 void (*ReleaseLongArrayElements)(JNIEnv*, jlongArray,
268 jlong*, jint);
269 void (*ReleaseFloatArrayElements)(JNIEnv*, jfloatArray,
270 jfloat*, jint);
271 void (*ReleaseDoubleArrayElements)(JNIEnv*, jdoubleArray,
272 jdouble*, jint);
273
274 void (*GetBooleanArrayRegion)(JNIEnv*, jbooleanArray,
275 jsize, jsize, jboolean*);
276 void (*GetByteArrayRegion)(JNIEnv*, jbyteArray,
277 jsize, jsize, jbyte*);
278 void (*GetCharArrayRegion)(JNIEnv*, jcharArray,
279 jsize, jsize, jchar*);
280 void (*GetShortArrayRegion)(JNIEnv*, jshortArray,
281 jsize, jsize, jshort*);
282 void (*GetIntArrayRegion)(JNIEnv*, jintArray,
283 jsize, jsize, jint*);
284 void (*GetLongArrayRegion)(JNIEnv*, jlongArray,
285 jsize, jsize, jlong*);
286 void (*GetFloatArrayRegion)(JNIEnv*, jfloatArray,
287 jsize, jsize, jfloat*);
288 void (*GetDoubleArrayRegion)(JNIEnv*, jdoubleArray,
289 jsize, jsize, jdouble*);
290
291 /* spec shows these without const; some jni.h do, some don't */
292 void (*SetBooleanArrayRegion)(JNIEnv*, jbooleanArray,
293 jsize, jsize, const jboolean*);
294 void (*SetByteArrayRegion)(JNIEnv*, jbyteArray,
295 jsize, jsize, const jbyte*);
296 void (*SetCharArrayRegion)(JNIEnv*, jcharArray,
297 jsize, jsize, const jchar*);
298 void (*SetShortArrayRegion)(JNIEnv*, jshortArray,
299 jsize, jsize, const jshort*);
300 void (*SetIntArrayRegion)(JNIEnv*, jintArray,
301 jsize, jsize, const jint*);
302 void (*SetLongArrayRegion)(JNIEnv*, jlongArray,
303 jsize, jsize, const jlong*);
304 void (*SetFloatArrayRegion)(JNIEnv*, jfloatArray,
305 jsize, jsize, const jfloat*);
306 void (*SetDoubleArrayRegion)(JNIEnv*, jdoubleArray,
307 jsize, jsize, const jdouble*);
308
309 jint (*RegisterNatives)(JNIEnv*, jclass, const JNINativeMethod*,
310 jint);
311 jint (*UnregisterNatives)(JNIEnv*, jclass);
312 jint (*MonitorEnter)(JNIEnv*, jobject);
313 jint (*MonitorExit)(JNIEnv*, jobject);
314 jint (*GetJavaVM)(JNIEnv*, JavaVM**);
315
316 void (*GetStringRegion)(JNIEnv*, jstring, jsize, jsize, jchar*);
317 void (*GetStringUTFRegion)(JNIEnv*, jstring, jsize, jsize, char*);
318
319 void* (*GetPrimitiveArrayCritical)(JNIEnv*, jarray, jboolean*);
320 void (*ReleasePrimitiveArrayCritical)(JNIEnv*, jarray, void*, jint);
321
322 const jchar* (*GetStringCritical)(JNIEnv*, jstring, jboolean*);
323 void (*ReleaseStringCritical)(JNIEnv*, jstring, const jchar*);
324
325 jweak (*NewWeakGlobalRef)(JNIEnv*, jobject);
326 void (*DeleteWeakGlobalRef)(JNIEnv*, jweak);
327
328 jboolean (*ExceptionCheck)(JNIEnv*);
329
330 jobject (*NewDirectByteBuffer)(JNIEnv*, void*, jlong);
331 void* (*GetDirectBufferAddress)(JNIEnv*, jobject);
332 jlong (*GetDirectBufferCapacity)(JNIEnv*, jobject);
333
334 /* added in JNI 1.6 */
335 jobjectRefType (*GetObjectRefType)(JNIEnv*, jobject);
336 };
可以看到在它其中定义了很多的函数指针，而通过这些定义，JNI层其实就获得了对DVM的引用，通过定义的这些函数指针，可以定位到虚拟机中的 JNI 函数表，从而实现JNI层在DVM中的函数调用。 所以，可以这样理解，其实JNIEnv，就是对DVM运行环境中C/C++函数的一个引用，而也正因为此，当C/C++想要在DVM中调用函数的时候，由于其是在DVM的环境中，所以它们必须通过JNIEnv* 这个参数来获得这些方法，之后才能够使用。 那么这个JNIEnv是什么时候产生的呢？ 当Android中第一个Java线程要调用本地的C/C++代码的时候，DVM就会为该线程产生一个JNIEnv*的指针。而每一个线程在和C/C++互相调用的时候，其对应的JNIEnv 也是相互独立。 嗯，结束。