AndroidJNI调用时缓存字段和方法ID示例

extern "C"
JNIEXPORT void JNICALL
Java_com_glumes_cppso_jnioperations_CacheFieldAndMethodOps_staticCacheField(JNIEnv *env, jobject instance, jobject animal) {
 static jfieldID fid = NULL; // 声明为 static 变量进行缓存
 // 两种方法都行
// jclass cls = env->GetObjectClass(animal);
 jclass cls = env->FindClass("com/glumes/cppso/model/Animal");
 jstring jstr;
 const char *c_str;
 // 从缓存中查找
 if (fid == NULL) {
  fid = env->GetFieldID(cls, "name", "Ljava/lang/String;");
  if (fid == NULL) {
   return;
  }
 } else {
  LOGD("field id is cached");
 }
 jstr = (jstring) env->GetObjectField(animal, fid);
 c_str = env->GetStringUTFChars(jstr, NULL);
 if (c_str == NULL) {
  return;
 }
 env->ReleaseStringUTFChars(jstr, c_str);
 jstr = env->NewStringUTF("new name");
 if (jstr == NULL) {
  return;
 }
 env->SetObjectField(animal, fid, jstr);
}

public class CacheFieldAndMethodOps extends BaseOperation {
 
 static {
  initCacheMethodId(); // 静态代码块中进行缓存
 }
 private static native void initCacheMethodId();
}

// 全局变量，作为缓存方法 id
jmethodID InstanceMethodCache;

// 初始化加载时缓存方法 id
extern "C"
JNIEXPORT void JNICALL
Java_com_glumes_cppso_jnioperations_CacheFieldAndMethodOps_initCacheMethodId(JNIEnv *env, jclass type) {
 jclass cls = env->FindClass("com/glumes/cppso/model/Animal");
 InstanceMethodCache = env->GetMethodID(cls, "getName", "()Ljava/lang/String;");
}

extern "C"
JNIEXPORT void JNICALL
Java_com_glumes_cppso_jnioperations_CacheFieldAndMethodOps_callCacheMethod(JNIEnv *env, jobject instance, jobject animal) {
 jstring name = (jstring) env->CallObjectMethod(animal, InstanceMethodCache);
 const char *c_name = env->GetStringUTFChars(name, NULL);
 LOGD("call cache method and value is %s", c_name);
}