1 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 2 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3 <html xmlns="http://www.w3.org/1999/xhtml"> 4 <head> 5 <meta http-equiv="Content-Type" content="text/xhtml;charset=UTF-8"/> 6 <meta http-equiv="X-UA-Compatible" content="IE=9"/> 7 <meta name="generator" content="Doxygen 1.8.5"/> 8 <title>NDK Programmer's Guide: hello-jni</title> 9 <link href="tabs.css" rel="stylesheet" type="text/css"/> 10 <script type="text/javascript" src="jquery.js"></script> 11 <script type="text/javascript" src="dynsections.js"></script> 12 <link href="navtree.css" rel="stylesheet" type="text/css"/> 13 <script type="text/javascript" src="resize.js"></script> 14 <script type="text/javascript" src="navtree.js"></script> 15 <script type="text/javascript"> 16 $(document).ready(initResizable); 17 $(window).load(resizeHeight); 18 </script> 19 <link href="doxygen.css" rel="stylesheet" type="text/css" /> 20 </head> 21 <body> 22 <div id="top"><!-- do not remove this div, it is closed by doxygen! --> 23 <div id="titlearea"> 24 <table cellspacing="0" cellpadding="0"> 25 <tbody> 26 <tr style="height: 56px;"> 27 <td style="padding-left: 0.5em;"> 28 <div id="projectname">NDK Programmer's Guide 29 </div> 30 </td> 31 </tr> 32 </tbody> 33 </table> 34 </div> 35 <!-- end header part --> 36 <!-- Generated by Doxygen 1.8.5 --> 37 </div><!-- top --> 38 <div id="side-nav" class="ui-resizable side-nav-resizable"> 39 <div id="nav-tree"> 40 <div id="nav-tree-contents"> 41 <div id="nav-sync" class="sync"></div> 42 </div> 43 </div> 44 <div id="splitbar" style="-moz-user-select:none;" 45 class="ui-resizable-handle"> 46 </div> 47 </div> 48 <script type="text/javascript"> 49 $(document).ready(function(){initNavTree('md_2__samples_sample--hellojni.html','');}); 50 </script> 51 <div id="doc-content"> 52 <div class="header"> 53 <div class="headertitle"> 54 <div class="title">hello-jni </div> </div> 55 </div><!--header--> 56 <div class="contents"> 57 <div class="textblock"><p>This sample provides a bare-bones look at a minimal 58 application built with the NDK.</p> 59 <h3>Android.mk</h3> 60 <p>The following two lines provide the name of the native source file, along 61 with the name of the shared library to build. The full name of the built 62 library is <code>libhello-jni.so</code>, but you should omit the 63 <code>lib</code> prefix and the <code>.so</code> extension.</p> 64 <pre class="fragment">LOCAL_SRC_FILES := hello-jni.c 65 LOCAL_MODULE := hello-jni 66 </pre><p>For more information on what this file does, and how to use it, see 67 the <a 68 href="./md_3__key__topics__building__chapter_1-section_8__android_8mk.html">Android.mk</a> section.</p> 69 <h3>Application.mk</h3> 70 <p>This line tells the build system the architecture against which to build. If 71 you don't specify anything, the build system defaults to armeabi.</p> 72 <pre class="fragment">APP_ABI := all 73 </pre><p>For more information on what this file does, and how to use it, see 74 the <a 75 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> 76 <h3>Java-side implementation</h3> 77 <p>This file calls a function to retrieve a string from the native side, then 78 displays it on the screen.</p> 79 <p>The source code contains three lines of particular interest to the NDK user. 80 They are presented here in the order in which they are used, rather than by 81 line order.</p> 82 <p>This function call loads the <code>.so</code> file upon application 83 startup.</p> 84 <pre class="fragment">System.loadLibrary("hello-jni"); 85 </pre><p>The <code>native</code> keyword in this method declaration tells the 86 DVM that the function is in the shared library (i.e., implemented on the native 87 side).</p> 88 <pre class="fragment">public native String stringFromJNI(); 89 </pre><p>The Android framework calls the function loaded and declared in the 90 previous steps, displaying the string on the screen.</p> 91 <pre class="fragment">tv.setText( stringFromJNI() ); 92 </pre><h3>C-side implementation</h3> 93 <p>This file contains a function that returns a string that the Java side 94 requested (see "Java-side implementation"). The function declaration is as 95 follows:</p> 96 <pre class="fragment">jstring Java_com_example_hellojni_HelloJni_stringFromJNI( 97 JNIEnv* env, 98 jobject x ) 99 </pre><p>This declaration corresponds to the native function declared in the 100 Java source code. The return type, <code>jstring</code>, is a data type defined 101 in the <a 102 href="http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/jniTOC.html 103 ">Java Native Interface Specification</a>. It is not actually a string, but a 104 pointer to a Java string.</p> 105 <p>After <code>jstring</code> comes the function name, which is based on the 106 Java function name and and the path to the file containing it. Construct it 107 according to the following rules:</p> 108 <ul> 109 <li>Prepend <code>Java_</code> to it.</li> 110 <li>Describe the filepath relative to the top-level source directory.</li> 111 <li>Use underscores in place of forward slashes.</li> 112 <li>Omit the <code>.java</code> file extension.</li> 113 <li>After the last underscore, append the function name.</li> 114 </ul> 115 <p>Thus, in this example, the function name 116 <code>Java_com_example_hellojni_HelloJni_stringFromJNI</code> refers to a Java 117 function called <code>stringFromJNI()</code>, which resides in 118 <code>hellojni/src/com/example/hellojni/HelloJni.java</code>.</p> 119 <p>Finally, <code>JNIEnv*</code> is the pointer to the VM, and 120 <code>jobject</code> is a pointer to the implicit this object passed from 121 the Java side.</p> 122 <p>The following line calls the VM API (*env), and passes it a return value: 123 that is, the string that the function on the Java side had requested. </p> 124 <pre class="fragment">return (*env)->NewStringUTF(env, "Hello from JNI ! 125 Compiled with ABI " ABI "."); </pre> </div></div><!-- contents --> 126 </div><!-- doc-content --> 127 <!-- start footer part --> 128 <div id="nav-path" class="navpath"><!-- id is needed for treeview function! --> 129 <ul> 130 <li class="footer">Generated on Wed Jun 25 2014 00:51:19 for NDK 131 Programmer's Guide by 132 <a href="http://www.doxygen.org/index.html"> 133 <img class="footer" src="doxygen.png" alt="doxygen"/></a> 1.8.5 </li> 134 </ul> 135 </div> 136 </body> 137 </html> 138
