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<div class="title">hello-jni </div> </div>
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<div class="textblock"><p>This sample provides a bare-bones look at a minimal
application built with the NDK.</p>
<h3>Android.mk</h3>
<p>The following two lines provide the name of the native source file, along
with the name of the shared library to build. The full name of the built
library is <code>libhello-jni.so</code>, but you should omit the
<code>lib</code> prefix and the <code>.so</code> extension.</p>
<pre class="fragment">LOCAL_SRC_FILES := hello-jni.c
LOCAL_MODULE := hello-jni
</pre><p>For more information on what this file does, and how to use it, see
the <a
href="./md_3__key__topics__building__chapter_1-section_8__android_8mk.html">Android.mk</a> section.</p>
<h3>Application.mk</h3>
<p>This line tells the build system the architecture against which to build. If
you don't specify anything, the build system defaults to armeabi.</p>
<pre class="fragment">APP_ABI := all
</pre><p>For more information on what this file does, and how to use it, see
the <a
href="./md_3__key__topics__building__a_p_p_l_i_c_a_t_i_o_n-_m_k.html">Application.mk</a> section.</p>
<h3>Java-side implementation</h3>
<p>This file calls a function to retrieve a string from the native side, then
displays it on the screen.</p>
<p>The source code contains three lines of particular interest to the NDK user.
They are presented here in the order in which they are used, rather than by
line order.</p>
<p>This function call loads the <code>.so</code> file upon application
startup.</p>
<pre class="fragment">System.loadLibrary("hello-jni");
</pre><p>The <code>native</code> keyword in this method declaration tells the
DVM that the function is in the shared library (i.e., implemented on the native
side).</p>
<pre class="fragment">public native String stringFromJNI();
</pre><p>The Android framework calls the function loaded and declared in the
previous steps, displaying the string on the screen.</p>
<pre class="fragment">tv.setText( stringFromJNI() );
</pre><h3>C-side implementation</h3>
<p>This file contains a function that returns a string that the Java side
requested (see "Java-side implementation"). The function declaration is as
follows:</p>
<pre class="fragment">jstring Java_com_example_hellojni_HelloJni_stringFromJNI(
JNIEnv* env,
jobject x )
</pre><p>This declaration corresponds to the native function declared in the
Java source code. The return type, <code>jstring</code>, is a data type defined
in the <a
href="http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/jniTOC.html
">Java Native Interface Specification</a>. It is not actually a string, but a
pointer to a Java string.</p>
<p>After <code>jstring</code> comes the function name, which is based on the
Java function name and and the path to the file containing it. Construct it
according to the following rules:</p>
<ul>
<li>Prepend <code>Java_</code> to it.</li>
<li>Describe the filepath relative to the top-level source directory.</li>
<li>Use underscores in place of forward slashes.</li>
<li>Omit the <code>.java</code> file extension.</li>
<li>After the last underscore, append the function name.</li>
</ul>
<p>Thus, in this example, the function name
<code>Java_com_example_hellojni_HelloJni_stringFromJNI</code> refers to a Java
function called <code>stringFromJNI()</code>, which resides in
<code>hellojni/src/com/example/hellojni/HelloJni.java</code>.</p>
<p>Finally, <code>JNIEnv*</code> is the pointer to the VM, and
<code>jobject</code> is a pointer to the implicit “this” object passed from
the Java side.</p>
<p>The following line calls the VM API (*env), and passes it a return value:
that is, the string that the function on the Java side had requested. </p>
<pre class="fragment">return (*env)->NewStringUTF(env, "Hello from JNI !
Compiled with ABI " ABI "."); </pre> </div></div><!-- contents -->
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