android NDK 实用学习（四）-类缓存

1，为什么需要类缓存：

答：由于频繁的查找类及类成员变量需要很大的时间与空间开销，可参考如下文章：

http://www.ibm.com/developerworks/cn/java/j-jni/

http://www.28im.com/java/a2379737.html

2，缓存时需要在java类使用static，如下：

 package com.dasea.test.core;
 public class TestSetData {
     // 主要是类ID和字段ID，方法ID的缓存
     static {
         OnNative();
     }

     public native static void OnNative();

     public boolean bData;
     public double  dData;
     public int     iData;
     public byte    cData;
     public String  sData;
     public byte    byteArr[];

     public int     intArr[];

     public TestSetData() {
         // TODO Auto-generated constructor stub
         bData = true;
         dData = 100.11;
         iData = 333;
         cData = 100;
         sData = "20150204";
         byteArr = new byte[10];
         for (int i = 0; i < byteArr.length; i++) {
             byteArr[i] = '2';
         }

         intArr = new int[10];
         for (int i = 0; i < intArr.length; i++) {
             intArr[i] = i * 10;
         }
     }
 }

3，c++端的相关实现代码：

①定义一个对应的结构体：

 struct JTestSetData{
     jclass   jtestSetData;

     jfieldID jbData;
     jfieldID jiData;
     jfieldID jdData;
     jfieldID jcData;
     jfieldID jsData;
     jfieldID jarrData;
 };

②定义对应类的变量：

struct JTestSetData gs_testSetDataMgr;

③实现缓存函数：

 JNIEXPORT void JNICALL Java_com_dasea_test_core_TestSetData_OnNative(
         JNIEnv* env, jobject obj){
     DEBUG_OUT("TestSetData native start!");

     InitTestSetData(env);

     DEBUG_OUT("TestSetData native end!");
 }

 bool InitTestSetData(JNIEnv* env){
     // 缓存类及其字段
     // 查找类里面的字段，并进行赋值

     // STEP 1/3 : Load the class id
     jclass jcSetDataMgr = env->FindClass("com/kq/rtk/core/TestSetData");

     // STEP 2/3 : Assign the ClassId as a Global Reference
     gs_testSetDataMgr.jtestSetData = (jclass) env->NewGlobalRef(jcSetDataMgr);

     jfieldID funB = env->GetFieldID(jcSetDataMgr, "bData", "Z");
     gs_testSetDataMgr.jbData = funB;
     gs_testSetDataMgr.jiData = env->GetFieldID(jcSetDataMgr, "iData", "I");
     gs_testSetDataMgr.jdData = env->GetFieldID(jcSetDataMgr, "dData", "D");
     gs_testSetDataMgr.jcData = env->GetFieldID(jcSetDataMgr, "cData", "B");
     gs_testSetDataMgr.jsData = env->GetFieldID(jcSetDataMgr, "sData", "Ljava/lang/String;");
     gs_testSetDataMgr.jarrData = env->GetFieldID(jcSetDataMgr, "intArr", "[I");

     // STEP 3/3 : Delete the no longer needed local reference
     env->DeleteLocalRef(jcSetDataMgr);

     return true;
 }

④使用缓存的类及成员：

java端接口：

public native void testPreCacheFun(TestSetData obj);

c++端实现：

 JNIEXPORT void JNICALL Java_com_dasea_test_core_RTKNativeManager_testPreCacheFun(
         JNIEnv* env, jobject obj, jobject jobj){
     DEBUG_OUT("testPreCache start!");

     if (NULL == gs_testSetDataMgr.jtestSetData) {
         DEBUG_OUT("No cache class!");
         if(false == InitTestSetData(env)){
             DEBUG_OUT("Cache failed!");
             return ;
         }
         DEBUG_OUT("Cache success!");
     }else{
         DEBUG_OUT("Has cache!");
     }

     env->SetBooleanField(jobj, gs_testSetDataMgr.jbData, false);
     env->SetDoubleField(jobj, gs_testSetDataMgr.jdData, 209.22);
     env->SetIntField(jobj, gs_testSetDataMgr.jiData, );
     env->SetByteField(jobj, gs_testSetDataMgr.jcData, );

     DEBUG_OUT("Set field succ!");

     char data[] = "jfkdsajfl";
     jstring sss = env->NewStringUTF(data);
     env->SetObjectField(jobj, gs_testSetDataMgr.jsData, sss);
     env->DeleteLocalRef(sss);

     // 获取Java中数组属性arrays的对象
     jintArray jint_arr = (jintArray)env->GetObjectField(jobj, gs_testSetDataMgr.jarrData);
     int arrInt[] = {};
     for(int i = ; i < ; ++i){
        arrInt[i] = +i;
     }
     env->SetIntArrayRegion(jint_arr, , , arrInt);

     DEBUG_OUT("testPreCache end!");
 }

⑤使用缓存类构造类对象：

      DEBUG_OUT("AllocObject object !");
      jmethodID initID = env->GetMethodID(gs_testGetDataMgr.jtestGetData, "<init>", "()V");
      jobject jresult = env->NewObject(gs_testGetDataMgr.jtestGetData, initID);