JNI学习笔记（2）

墨蓝 2022-08-19 12:22 181阅读 0赞

# JavaVM接口 #

第一种方式，在加载动态链接库的时候，JVM会调用JNI\_OnLoad(JavaVM\* jvm, void\* reserved)（如果定义了该函数）。第一个参数会传入JavaVM指针。一般都在这个时候保存一个static的JavaVM \*jvm，这个jvm指针在进程内是可以放心共享的。

第二种方式，在native code中调用JNI\_CreateJavaVM(&jvm, (void\*\*)&env, &vm\_args)可以得到JavaVM指针。

两种情况下，都可以用全局变量，比如JavaVM\* g\_jvm来保存获得的指针以便在任意上下文中使用。

Android系统是利用第二种方式Invocation interface来创建JVM的。

# JNIEnv接口 #

JNI开发最常见的错误就是滥用了JNIEnv接口。需要强调的是JNIEnv是跟线程相关的。sdk文档中强调了do not cache JNIEnv\*，要用的时候在不同线程中再通过JavaVM \*jvm的方法来获取与当前线程相关的JNIEnv\*。

在native method中，JNIEnv作为第一个参数传入。那么在JNIEnv不作为参数传入的时候，该如何获得它？JNI提供了两个函数：（\*jvm)->AttachCurrentThread(jvm, (void\*\*)&env, NULL)和(\*jvm)->GetEnv(jvm, (void\*\*)&env, JNI\_VERSION\_1\_2)。两个函数都利用JavaVM接口获得JNIEnv接口，上面已经讲到如何获得JavaVM接口。当创建的线程需要获取JNIEnv\*的时候，最好在刚创建的时候调用一次AttachCurrentThread，最好还是不要缓存这个JNIEnv\*，每次需要的时候通过JavaVM\*获取，不要忘记线程结束的时候执行DettachCurrentThread。

JNI规范也说明，可以将获得JNIEnv封装成一个函数。

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转贴：http://blog.csdn.net/lirunfa/article/details/8030094

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