blocks|key|1237033|text|不，不幸的是这不可能。每个进程只能有一个JVM，而且您已经在一个JVM进程中。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1237034|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

No, unfortunately its not possible. You can only have one JVM per process, and you're already in a JVM process.

blocks|key|5021996|text|我看不出Java-->C-->Java有什么意义。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5021997|如果可以从Java调用C，那么可以在C函数返回后从Java调用Java。|5021998|Excelsion+xFunction是一个用于从Java调用C的简单而可靠的库。它处理那些奇怪的JNI东西，给你一个更简单的界面。|offset|length|5021999|entityMap|0|LINK|mutability|MUTABLE|url|http://www.excelsior-usa.com/xfunction.html^0|0|0|0|J|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Q|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@$F|S|G|T|1|U]]|A|$]]|$1|H|3|-4|5|6|7|V|8|@]|9|@]|A|$]]]|I|$J|$5|K|L|M|A|$N|O]]]]

I can not see the point in that Java-->C-->Java.

If you can call C from Java, then you can call Java from Java after your C function returns. 

<a href="http://www.excelsior-usa.com/xfunction.html" rel="nofollow">Excelsion xFunction</a> is an easy and reliable library for calling C from Java. It handles the weird JNI stuff giving you a simpler interface.

blocks|key|3553964|text|为什么需要创建第二个JVM？您不能创建第二个JVM，但可以从C代码访问Java类。参见Accessing+Java+Objects。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3553965|entityMap|0|LINK|mutability|MUTABLE|url|http://download.oracle.com/javase/1.5.0/docs/guide/jni/spec/design.html#wp1253^0|17|M|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Why do you need to create a second JVM? You can't create a second JVM, but you can access Java classes from C code. See <a href="http://download.oracle.com/javase/1.5.0/docs/guide/jni/spec/design.html#wp1253" rel="nofollow">Accessing Java Objects</a>.

blocks|key|1237216|text|我不明白为什么有些人认为这是无用的。事实上，当您进入->游戏编程时，Java+->+C->+Java场景是非常常见的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1237217|1237218|安卓应用程序从一些用Java语言编写的样板代码开始(使用软件开发工具包)|ordered-list-item|1237219|启动一个新线程来运行游戏的主要逻辑，它是用C/+C%2B%2B编写的，以提高效率(并且更容易访问OpenGL+ES)|offset|length|1237220|The+C/+C%2B%2B代码调用所需functionalities|1237221|的一些Java方法|1237222|之所以有必要使用SDK+(Java)，是因为大多数Android都是用Java语言提供的！NDK+(C+/+C%2B%2B)只提供了一个精简的Linux环境(可以把它想象成“标准C库”%2B“标准C%2B%2B库”%2B“精简版的Linux系统库”)。例如，如果您想要将您的应用程序与AdMob+(Google+Ads)或In-app+Billing+(Google+Payment)集成，或者如果您想访问设备的内置摄像头，则必须从游戏逻辑(它是用C/C%2B%2B编写的)中调用Java方法。|1237223|Java+->+C部分|1237224|编写Java类。声明一个方法native|style|CODE|1237225|$+ls+-F1
classes/
jni/
src/

$+nano+src/com/example/jcj/Test.java|code-block|syntax|javascript|1237226|Test.java：|1237227|package+com.example.jcj;

public+class+Test+{
++++public+static+void+main(String[]+args)+{
++++++++doSomethingInC();
++++}

++++public+static+native+void+doSomethingInC();

++++public+static+void+doSomethingInJava()+{
++++++++System.out.println("Done+something+in+Java!");
++++}

++++static+{
++++++++System.loadLibrary("hello_jcj");
++++}
}|1237228|调用"Java编译器“(javac)，然后调用"C头和存根文件生成器”(javah)：|1237229|$+javac+-sourcepath+src+-d+classes+src/com/example/jcj/Test.java

$+javah+-classpath+classes+-d+jni+com.example.jcj.Test

$+ls+-F1+jni/
com_example_jcj_Test.h|1237230|实现C部分：|1237231|$+nano+jni/com_example_jcj_Test.c|1237232|com_example_jcj_Test.c：|1237233|#include+"com_example_jcj_Test.h"
#include+<stdio.h>

/*
+*+Class:+++++com_example_jcj_Test
+*+Method:++++doSomethingInC
+*+Signature:+()V
+*/
JNIEXPORT+void+JNICALL
Java_com_example_jcj_Test_doSomethingInC(JNIEnv*+env,+jclass+clazz)
{
++++printf("Done+something+in+C!\n");
}|1237234|编译C源代码(Windows)：|1237235|>+REM+TODO:+Add+this!|1237236|编译C源代码(Linux)：|1237237|$+gcc+-I{JAVA_HOME}/include
++++++-shared+\
++++++-o+libhello_jcj.so+\
++++++jni/com_example_jcj_Test.c

$+ls+-F1
classes/
jni/
libhello_jcj.so*
src/|1237238|编译C源代码(Mac+OS+X)：|1237239|$+gcc+-I${JAVA_HOME}/include+\
++++++-shared+\
++++++-o+libhello_jcj.jnilib+\
++++++jni/com_example_jcj_Test.c

$+ls+-F1
classes/
jni/
libhello_jcj.jnilib*
src/|1237240|运行Java应用程序：|1237241|$+java+-classpath+classes+com.example.jcj.Test
Done+something+in+C!|1237242|C+->+Java部分|1237243|由于您是在Java语言(而不是C/+C%2B%2B)中启动程序的，因此已经创建了一个JVM。您不需要在第二部分(C+->+Java部分)中创建另一个，您只需要重用已经创建的一个。有两种方法可以做到：|1237244|1237245|通过使用由来自第一部分(Java+->+C部分)的JNI调用提供的Java环境指针(JNIEnv*)+|1237246|通过使用在JNI_OnLoad()|1237247|期间保存的Java虚拟机指针(JavaVM*|1237248|(答案还没有回答完，我稍后再回答。如果你喜欢，请给我投票鼓励我。谢谢！)|BOLD|1237249|参考文献：|1237250|1237251|JNI+Functions+-+Java+Native+Interface+Specification|unordered-list-item|1237252|The+Invocation+API+-+Java+Native+Interface+Specification|1237253|JNI+Tips+%7C+Android+Developers|1237254|1237255|entityMap|0|LINK|mutability|MUTABLE|url|http://www.khronos.org/opengles/|1|http://developer.android.com/sdk/index.html|2|http://developer.android.com/tools/sdk/ndk/index.html|3|https://developers.google.com/mobile-ads-sdk/|4|http://developer.android.com/google/play/billing/index.html|5|http://docs.oracle.com/javase/6/docs/technotes/guides/jni/spec/functions.html|6|http://docs.oracle.com/javase/6/docs/technotes/guides/jni/spec/invocation.html|7|http://developer.android.com/training/articles/perf-jni.html^0|0|0|0|18|9|0|0|0|0|8|A|1|19|D|2|3M|I|3|45|V|4|0|0|E|6|0|0|0|0|C|5|10|5|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|16|7|0|5|C|0|F|7|0|0|10|0|0|0|0|1F|5|0|0|1K|6|0|0|T|7|0|0^^$0|@$1|2|3|4|5|6|7|37|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|38|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|39|8|@]|9|@]|A|$]]|$1|F|3|G|5|E|7|3A|8|@]|9|@$H|3B|I|3C|1|3D]]|A|$]]|$1|J|3|K|5|E|7|3E|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|3F|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|3G|8|@]|9|@$H|3H|I|3I|1|3J]|$H|3K|I|3L|1|3M]|$H|3N|I|3O|1|3P]|$H|3Q|I|3R|1|3S]]|A|$]]|$1|P|3|Q|5|6|7|3T|8|@]|9|@]|A|$]]|$1|R|3|S|5|6|7|3U|8|@$H|3V|I|3W|T|U]]|9|@]|A|$]]|$1|V|3|W|5|X|7|3X|8|@]|9|@]|A|$Y|Z]]|$1|10|3|11|5|6|7|3Y|8|@]|9|@]|A|$]]|$1|12|3|13|5|X|7|3Z|8|@]|9|@]|A|$Y|Z]]|$1|14|3|15|5|6|7|40|8|@$H|41|I|42|T|U]|$H|43|I|44|T|U]]|9|@]|A|$]]|$1|16|3|17|5|X|7|45|8|@]|9|@]|A|$Y|Z]]|$1|18|3|19|5|6|7|46|8|@]|9|@]|A|$]]|$1|1A|3|1B|5|X|7|47|8|@]|9|@]|A|$Y|Z]]|$1|1C|3|1D|5|6|7|48|8|@]|9|@]|A|$]]|$1|1E|3|1F|5|X|7|49|8|@]|9|@]|A|$Y|Z]]|$1|1G|3|1H|5|6|7|4A|8|@]|9|@]|A|$]]|$1|1I|3|1J|5|X|7|4B|8|@]|9|@]|A|$Y|Z]]|$1|1K|3|1L|5|6|7|4C|8|@]|9|@]|A|$]]|$1|1M|3|1N|5|X|7|4D|8|@]|9|@]|A|$Y|Z]]|$1|1O|3|1P|5|6|7|4E|8|@]|9|@]|A|$]]|$1|1Q|3|1R|5|X|7|4F|8|@]|9|@]|A|$Y|Z]]|$1|1S|3|1T|5|6|7|4G|8|@]|9|@]|A|$]]|$1|1U|3|1V|5|X|7|4H|8|@]|9|@]|A|$Y|Z]]|$1|1W|3|1X|5|6|7|4I|8|@]|9|@]|A|$]]|$1|1Y|3|1Z|5|6|7|4J|8|@]|9|@]|A|$]]|$1|20|3|-4|5|6|7|4K|8|@]|9|@]|A|$]]|$1|21|3|22|5|E|7|4L|8|@$H|4M|I|4N|T|U]]|9|@]|A|$]]|$1|23|3|24|5|E|7|4O|8|@$H|4P|I|4Q|T|U]]|9|@]|A|$]]|$1|25|3|26|5|6|7|4R|8|@$H|4S|I|4T|T|U]]|9|@]|A|$]]|$1|27|3|28|5|6|7|4U|8|@$H|4V|I|4W|T|29]]|9|@]|A|$]]|$1|2A|3|2B|5|6|7|4X|8|@]|9|@]|A|$]]|$1|2C|3|-4|5|6|7|4Y|8|@]|9|@]|A|$]]|$1|2D|3|2E|5|2F|7|4Z|8|@]|9|@$H|50|I|51|1|52]]|A|$]]|$1|2G|3|2H|5|2F|7|53|8|@]|9|@$H|54|I|55|1|56]]|A|$]]|$1|2I|3|2J|5|2F|7|57|8|@]|9|@$H|58|I|59|1|5A]]|A|$]]|$1|2K|3|-4|5|6|7|5B|8|@]|9|@]|A|$]]|$1|2L|3|-4|5|6|7|5C|8|@]|9|@]|A|$]]]|2M|$2N|$5|2O|2P|2Q|A|$2R|2S]]|2T|$5|2O|2P|2Q|A|$2R|2U]]|2V|$5|2O|2P|2Q|A|$2R|2W]]|2X|$5|2O|2P|2Q|A|$2R|2Y]]|2Z|$5|2O|2P|2Q|A|$2R|30]]|31|$5|2O|2P|2Q|A|$2R|32]]|33|$5|2O|2P|2Q|A|$2R|34]]|35|$5|2O|2P|2Q|A|$2R|36]]]]

I can't see why some people consider this useless. In fact, the Java -> C -> Java scenario is very common when you get into Android game programming:

<ol>
<li>The Android app starts from some boilerplate code written in Java (using the SDK)</li>
<li>A new thread is started to run the main logic of the game, which is written in C / C++ for efficiency (and the easier access to <a href="http://www.khronos.org/opengles/" rel="nofollow">OpenGL ES</a>)</li>
<li>The C / C++ code calls some Java methods for the required functionalities</li>
</ol>

The reason why it is necessary to use the <a href="http://developer.android.com/sdk/index.html" rel="nofollow">SDK (Java)</a> is that most of the Android APIs are provided only in Java! The <a href="http://developer.android.com/tools/sdk/ndk/index.html" rel="nofollow">NDK (C / C++)</a> only provides a slim Linux environment (think it as "Standard C Library" + "Standard C++ Library" + "Cutdown Version of Linux System Libraries"). For example, if you want to integrate your app with <a href="https://developers.google.com/mobile-ads-sdk/" rel="nofollow">AdMob (Google Ads)</a>, or <a href="http://developer.android.com/google/play/billing/index.html" rel="nofollow">In-app Billing (Google Payment)</a>, or if you want to access the built-in camera of the device, you have to invoke Java methods from the game logic (which is written is C / C++!).

<h2>The Java -> C Part</h2>

Write a Java class. Declare one method <code>native</code>:

<pre><code>$ ls -F1
classes/
jni/
src/

$ nano src/com/example/jcj/Test.java
</code></pre>

Test.java:

<pre><code>package com.example.jcj;

public class Test {
 public static void main(String[] args) {
 doSomethingInC();
 }

 public static native void doSomethingInC();

 public static void doSomethingInJava() {
 System.out.println("Done something in Java!");
 }

 static {
 System.loadLibrary("hello_jcj");
 }
}
</code></pre>

Invoke the "Java compiler" (<code>javac</code>), followed by the "C header and stub file generator" (<code>javah</code>):

<pre><code>$ javac -sourcepath src -d classes src/com/example/jcj/Test.java

$ javah -classpath classes -d jni com.example.jcj.Test

$ ls -F1 jni/
com_example_jcj_Test.h
</code></pre>

Implement the C part:

<pre><code>$ nano jni/com_example_jcj_Test.c
</code></pre>

com_example_jcj_Test.c:

<pre><code>#include "com_example_jcj_Test.h"
#include &lt;stdio.h&gt;

/*
 * Class: com_example_jcj_Test
 * Method: doSomethingInC
 * Signature: ()V
 */
JNIEXPORT void JNICALL
Java_com_example_jcj_Test_doSomethingInC(JNIEnv* env, jclass clazz)
{
 printf("Done something in C!\n");
}
</code></pre>

Compile the C source (Windows):

<pre><code>&gt; REM TODO: Add this!
</code></pre>

Compile the C source (Linux):

<pre><code>$ gcc -I{JAVA_HOME}/include
 -shared \
 -o libhello_jcj.so \
 jni/com_example_jcj_Test.c

$ ls -F1
classes/
jni/
libhello_jcj.so*
src/
</code></pre>

Compile the C source (Mac OS X):

<pre><code>$ gcc -I${JAVA_HOME}/include \
 -shared \
 -o libhello_jcj.jnilib \
 jni/com_example_jcj_Test.c

$ ls -F1
classes/
jni/
libhello_jcj.jnilib*
src/
</code></pre>

Run the Java application:

<pre><code>$ java -classpath classes com.example.jcj.Test
Done something in C!
</code></pre>

<h2>The C -> Java Part</h2>

Since you started your program in Java (instead of in C / C++), a JVM is already created. You don't need to create another one in the second part (C -> Java part), you just need to reuse the one already created. There are 2 ways to do it:

<ol>
<li>By using the Java Environment pointer (<code>JNIEnv*</code>) provided by the JNI call from the first part (Java -> C part)</li>
<li>By using the Java Virtual Machine pointer (<code>JavaVM*</code>) saved during <code>JNI_OnLoad()</code></li>
</ol>

(The answer is not finished and I will finish it later. If you like it, please vote me up for encouragement. Thanks!)

References:

<ul>
<li><a href="http://docs.oracle.com/javase/6/docs/technotes/guides/jni/spec/functions.html" rel="nofollow">JNI Functions - Java Native Interface Specification</a></li>
<li><a href="http://docs.oracle.com/javase/6/docs/technotes/guides/jni/spec/invocation.html" rel="nofollow">The Invocation API - Java Native Interface Specification</a></li>
<li><a href="http://developer.android.com/training/articles/perf-jni.html" rel="nofollow">JNI Tips | Android Developers</a></li>
</ul>

I want to call a C funcion from Java using JNI. And in the C function, I want to create a JVM and call some Java objects. When I try to create the JVM, JNI_CreateJavaVM returns -1. 

So, I want to know if it is possible to do this. The C code is compiled to create a .so file (in linux), and the Java code calls the function in the .so file.

Any example doing Java->C->Java will be useful.

Thanks.

Java calling C calling Java

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我想使用JNI从Java调用一个C函数。在C函数中，我想创建一个JVM并调用一些Java对象。当我尝试创建JVM时，JNI_CreateJavaVM返回-1。所以，我想知道是否可以做到这一点。编译C代码以创建一个.so文件(在linux中)，然后Java代码调用.so文件中的函数。任何执行Java->C->Java的示例都会很有用。谢谢。

问Java调用C调用Java
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