ch05.html

java2高级编程

</DIV>
<DIV>
<H5 CLASS="B">
<A NAME="pgfId-1087494"></A>Run the Example</H5>
<P CLASS="Body">
<A NAME="pgfId-1087495"></A>To run the example, the Java virtual machine needs to find the native JNI library that was created. To do this, set the library path to the current directory as follows: </P>
<DIV>
<H6 CLASS="D">
<A NAME="pgfId-1087496"></A><EM CLASS="D-Code">
Unix or Linux:</EM>
</H6>
<PRE CLASS="CODE"><A NAME="pgfId-1087497"></A>LD_LIBRARY_PATH=.</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087498"></A>export LD_LIBRARY_PATH</PRE>
</DIV>
<DIV>
<H6 CLASS="D">
<A NAME="pgfId-1087499"></A><EM CLASS="D-Code">
Windows NT/2000/95:</EM>
</H6>
<PRE CLASS="CODE"><A NAME="pgfId-1087500"></A>set PATH=%path%;.</PRE>
<P CLASS="Body">
<A NAME="pgfId-1087501"></A>With the library path properly specified for your platform, invoke the program as you normally would with the interpreter command: </P>
<PRE CLASS="CODE"><A NAME="pgfId-1087502"></A>java ReadFile</PRE>
</DIV>
</DIV>
</DIV>
<DIV>
<H4 CLASS="A">
<A NAME="pgfId-1087504"></A><A NAME="22670"></A>Strings and Arrays</H4>
<P CLASS="Body">
<A NAME="pgfId-1087505"></A>This section explains how to pass string and array data between a program written in the Java programming language and a program written in another language.</P>
<DIV>
<H5 CLASS="B">
<A NAME="pgfId-1087506"></A>Passing Strings</H5>
<P CLASS="Body">
<A NAME="pgfId-1087509"></A>The <EM CLASS="CODE">
String</EM>
 object in the Java programming language, which is represented as <EM CLASS="CODE">
jstring</EM>
<A NAME="marker-1087507"></A><A NAME="marker-1087508"></A> in JNI, is a 16-bit unicode string. In C a string is by default constructed from 8-bit characters. So, to access a Java String object passed to a C or C++ function or return a C or C++ string to a Java method, you need to use JNI conversion functions in your native method implementation. </P>
<P CLASS="Body">
<A NAME="pgfId-1087511"></A>The <EM CLASS="CODE">
GetStringUTFChar</EM>
<A NAME="marker-1087510"></A> function retrieves 8-bit characters from a 16-bit <EM CLASS="CODE">
jstring</EM>
 using the <A NAME="marker-1087512"></A>Unicode Transformation Format (UTF). UTF represents Unicode as a string of 8- or 16-bit characters without losing any information. The third parameter <EM CLASS="CODE">
GetStringUTFChar</EM>
 returns the result <EM CLASS="CODE">
JNI_TRUE</EM>
<A NAME="marker-1087513"></A> if it made a local copy of the <EM CLASS="CODE">
jstring,</EM>
 or it returns <EM CLASS="CODE">
JNI_FALSE</EM>
<A NAME="marker-1087514"></A> otherwise. </P>
<DIV>
<H6 CLASS="D">
<A NAME="pgfId-1087515"></A><EM CLASS="D-Code">
C Version:</EM>
</H6>
<PRE CLASS="CODE"><A NAME="pgfId-1087516"></A>(*env)-&gt;GetStringUTFChars(env, name, iscopy)</PRE>
</DIV>
<DIV>
<H6 CLASS="D">
<A NAME="pgfId-1087517"></A><EM CLASS="D-Code">
C++ Version:</EM>
</H6>
<PRE CLASS="CODE"><A NAME="pgfId-1087518"></A>env-&gt;GetStringUTFChars(name, iscopy)</PRE>
<P CLASS="Body">
<A NAME="pgfId-1087519"></A>The following C JNI function converts an array of C characters to a <EM CLASS="CODE">
jstring</EM>
: </P>
<PRE CLASS="CODE-caption"><A NAME="pgfId-1087521"></A>//API Ref: <A NAME="36311"></A>jstring NewStringUTF(JNIEnv *env, const char *bytes)</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087524"></A><A NAME="marker-1087522"></A><A NAME="NewStringUTF function"></A>(*env)-&gt;NewStringUTF(env, lastfile)</PRE>
<P CLASS="Body">
<A NAME="pgfId-1087525"></A>The example below converts the <EM CLASS="CODE">
lastfile[80]</EM>
 C character array to a <EM CLASS="CODE">
jstring</EM>
, which is returned to the calling Java method: </P>
<PRE CLASS="CODE"><A NAME="pgfId-1087526"></A>static char lastfile[80];

JNIEXPORT jstring JNICALL Java_ReadFile_lastFile
  (JNIEnv *env, jobject jobj) {</PRE>
<PRE CLASS="CODE-caption"><A NAME="pgfId-1087528"></A>//API Ref: <A NAME="52421"></A>jstring NewStringUTF(JNIEnv *env, const char *bytes)</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087530"></A>   <A NAME="marker-1087529"></A>return((*env)-&gt;NewStringUTF(env, lastfile));
}</PRE>
<P CLASS="Body">
<A NAME="pgfId-1087531"></A>To let the Java virtual machine know you are finished with the UTF representation, call the <EM CLASS="CODE">
ReleaseStringUTFChars</EM>
<A NAME="marker-1087532"></A> conversion function as shown below. The second argument is the original <EM CLASS="CODE">
jstring</EM>
 value used to construct the UTF representation, and the third argument is the reference to the local representation of that <EM CLASS="CODE">
String</EM>
. </P>
<PRE CLASS="CODE"><A NAME="pgfId-1087533"></A>(*env)-&gt;ReleaseStringUTFChars(env, name, mfile);</PRE>
<P CLASS="Body">
<A NAME="pgfId-1087534"></A>If your native code can work with Unicode without needing the intermediate UTF representation, call the <EM CLASS="CODE">
GetStringChars</EM>
<A NAME="marker-1087535"></A> function to retrieve the Unicode string, and release the reference with a call to <EM CLASS="CODE">
ReleaseStringChars</EM>
: </P>
<PRE CLASS="CODE"><A NAME="pgfId-1087536"></A>JNIEXPORT jbyteArray JNICALL Java_ReadFile_loadFile
  (JNIEnv * env, jobject jobj, jstring name) {
    caddr_t m;
    jbyteArray jb;
    struct stat finfo;
    jboolean iscopy;</PRE>
<PRE CLASS="CODE-caption"><A NAME="pgfId-1087538"></A>//API Ref: <A NAME="59207"></A>const jchar *GetStringChars(JNIEnv *env, jstring string, 
                                            jboolean *iscopy)</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087541"></A>    <A NAME="marker-1087539"></A><A NAME="GetStringChars function"></A>const jchar *mfile = (*env)-&gt;GetStringChars(env, name, &amp;iscopy);
  //...</PRE>
<PRE CLASS="CODE-caption"><A NAME="pgfId-1087543"></A>//API Ref: <A NAME="40841"></A>void ReleaseStringChars(JNIEnv *env, jstring string, 
                                        const jchar *chars)</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087546"></A>    <A NAME="marker-1087544"></A><A NAME="ReleaseStringChars function"></A>(*env)-&gt;ReleaseStringChars(env, name, mfile);</PRE>
</DIV>
</DIV>
<DIV>
<H5 CLASS="B">
<A NAME="pgfId-1087547"></A>Passing Arrays</H5>
<P CLASS="Body">
<A NAME="pgfId-1087550"></A><A NAME="marker-1087548"></A><A NAME="marker-1087549"></A>In the example presented in the last section, the <EM CLASS="CODE">
loadFile</EM>
 native method returns the contents of a file in a byte array, which is a primitive type in the Java programming language. You can retrieve and create primitive types in the Java programming language by calling the appropriate <EM CLASS="CODE">
TypeArray</EM>
<A NAME="marker-1087551"></A> function. </P>
<P CLASS="Body">
<A NAME="pgfId-1087553"></A>For example, to create a new array of floats, call <EM CLASS="CODE">
NewFloatArray</EM>
<A NAME="marker-1087552"></A>, or to create a new array of bytes, call <EM CLASS="CODE">
NewByteArray</EM>
<A NAME="marker-1087554"></A>. This naming scheme extends to retrieving elements from, adding elements to, and changing elements in the array. To get elements from an array of bytes, call <EM CLASS="CODE">
GetByteArrayElements</EM>
<A NAME="marker-1087555"></A>. To add elements to or change elements in the array, call <EM CLASS="CODE">
Set&lt;type&gt;ArrayElements</EM>
<A NAME="marker-1087556"></A>. </P>
<P CLASS="Body">
<A NAME="pgfId-1087557"></A>The <EM CLASS="CODE">
GetByteArrayElements</EM>
 function affects the entire array. To work on a portion of the array, call <EM CLASS="CODE">
GetByteArrayRegion</EM>
<A NAME="marker-1087558"></A> instead. There is only a <EM CLASS="CODE">
Set&lt;type&gt;ArrayRegion</EM>
<A NAME="marker-1087559"></A> function for changing array elements. However the region could be of size <EM CLASS="CODE">
array.length</EM>
, which is equivalent to the nonexistent <EM CLASS="CODE">
Set&lt;type&gt;ArrayElements</EM>
.<A NAME="marker-1087560"></A> </P>
<TABLE>
<TR>
<TH ROWSPAN="1" COLSPAN="1">
<P CLASS="TCH">
<A NAME="pgfId-1087563"></A><EM CLASS="Bold">
Native Code Type</EM>
</P>
</TH>
<TH ROWSPAN="1" COLSPAN="1">
<P CLASS="TCH">
<A NAME="pgfId-1087565"></A><EM CLASS="Bold">
Functions Used</EM>
</P>
</TH>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TB1">
<A NAME="pgfId-1087568"></A><EM CLASS="CODE">
jboolean</EM>
<A NAME="marker-1087567"></A></P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TB1">
<A NAME="pgfId-1087570"></A><EM CLASS="CODE">
NewBooleanArray</EM>
</P>
<P CLASS="TB">
<A NAME="pgfId-1087571"></A><EM CLASS="CODE">
GetBooleanArrayElements </EM>
</P>
<P CLASS="TB">
<A NAME="pgfId-1087572"></A><EM CLASS="CODE">
GetBooleanArrayRegion/SetBooleanArrayRegion</EM>
</P>
<P CLASS="TB">
<A NAME="pgfId-1087573"></A><EM CLASS="CODE">
ReleaseBooleanArrayRegion </EM>
</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TB">
<A NAME="pgfId-1087576"></A><EM CLASS="CODE">
jbyte</EM>
<A NAME="marker-1087575"></A></P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TB">
<A NAME="pgfId-1087578"></A><EM CLASS="CODE">
NewByteArray </EM>
</P>
<P CLASS="TB">
<A NAME="pgfId-1087579"></A><EM CLASS="CODE">
GetByteArrayElements</EM>
</P>
<P CLASS="TB">
<A NAME="pgfId-1087580"></A><EM CLASS="CODE">
GetByteArrayRegion/SetByteArrayRegion</EM>
</P>
<P CLASS="TB">
<A NAME="pgfId-1087581"></A><EM CLASS="CODE">
ReleaseByteArrayRegion </EM>
</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TB">
<A NAME="pgfId-1087584"></A><EM CLASS="CODE">
jchar</EM>
<A NAME="marker-1087583"></A></P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TB">
<A NAME="pgfId-1087586"></A><EM CLASS="CODE">
NewCharArray </EM>
</P>
<P CLASS="TB">
<A NAME="pgfId-1087587"></A><EM CLASS="CODE">
GetCharArrayElements</EM>
</P>
<P CLASS="TB">
<A NAME="pgfId-1087588"></A><EM CLASS="CODE">
GetCharArrayRegion/SetCharArrayRegion</EM>
</P>
<P CLASS="TB">
<A NAME="pgfId-1087589"></A><EM CLASS="CODE">
ReleaseCharArrayRegion </EM>
</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TB">
<A NAME="pgfId-1087592"></A><EM CLASS="CODE">
jdouble</EM>
<A NAME="marker-1087591"></A></P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TB">
<A NAME="pgfId-1087594"></A><EM CLASS="CODE">
NewDoubleArray </EM>
</P>
<P CLASS="TB">
<A NAME="pgfId-1087595"></A><EM CLASS="CODE">
GetDoubleArrayElements</EM>
</P>
<P CLASS="TB">
<A NAME="pgfId-1087596"></A><EM CLASS="CODE">
GetDoubleArrayRegion/SetDoubleArrayRegion</EM>
</P>
<P CLASS="TB">
<A NAME="pgfId-1087597"></A><EM CLASS="CODE">
ReleaseDoubleArrayRegion </EM>
</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TB">
<A NAME="pgfId-1087600"></A><EM CLASS="CODE">
jfloat</EM>
<A NAME="marker-1087599"></A></P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TB">
<A NAME="pgfId-1087602"></A><EM CLASS="CODE">
NewFloatArray </EM>
</P>
<P CLASS="TB">
<A NAME="pgfId-1087603"></A><EM CLASS="CODE">
GetFloatArrayElements</EM>