ch10.htm

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&nbsp;&nbsp;&nbsp;&nbsp;public String getRow(int row)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;throws DBException
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if ( !query )
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;throw
new DBException(&quot;No active query&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else if ( row
&gt;= nRows )<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;throw
new DBException(&quot;Row out of bounds&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;String buildResult
= new String(&quot;&quot;);<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;for ( int x =
0; x &lt; nCols; x++ )<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;buildResult
+= (result[row])[x] + &quot; |&quot;;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return buildResult;
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;/**<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * Retreive the contents of a column.
<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * @param row, col is the column to retreive
<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * @exception DBException if invalid row
or column<BR>
&nbsp;&nbsp;&nbsp;&nbsp; */<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public String getColumn(int row, int col)
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;throws DBException
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if ( !query )
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;throw
new DBException(&quot;No active query&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else if ( row
&gt;= nRows )<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;throw
new DBException(&quot;Row out of bounds&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else if ( col
&gt;= nCols )<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;throw
new DBException(&quot;Column out of bounds&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return result[row][col];
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public void allDone(String str)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(str);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;/**<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * Display the contents of the statement
<BR>
&nbsp;&nbsp;&nbsp;&nbsp; */<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public String toString()<BR>
&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;String s = new
String();<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if ( query ==
false )<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;s
+= sqlStmt;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;try
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;for
( int x = 0; x &lt; nRows; x++ )<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;s
+= getRow(x) + &quot;\n&quot;;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;catch
(DBException de)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(de);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return s;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
}</TT>
</BLOCKQUOTE>
<HR>
<P>
The SQLStmt class has public methods to enable extracting query
data in an orderly manner: <TT>numRows()</TT>,
<TT>numCols()</TT>, <TT>getRow()</TT>,
and <TT>getColumn()</TT>. SQLStmts
are two-way objects-they hold both the input and the output data.
The output is contained in a two-dimensional array of Strings.
This scheme forces the database interface library to translate
all table columns into String format.
<P>
A new exception has been defined with this class. It can be found
on this book's CD-ROM in the file DBException.java. This exception
is thrown by the SQLStmt class when a request is made, but no
query has been attempted. The Database class, discussed in the
following paragraphs, also throws DBExceptions.
<P>
The method <TT>allDone()</TT> will
be called by the native library and gives the library a convenient
way to print. It serves no other purpose.
<P>
Although the SQLStmt class contains all the database information,
it does not actually interface with the native library. For this
task, the Database class is used. This class is much simpler than
the data container; its chief role is to interface with the native
methods. Listing 10.5 shows the Database class.
<HR>
<BLOCKQUOTE>
<B>Listing 10.5. The Database class.<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>import java.lang.*;<BR>
import java.io.*;<BR>
import SQLStmt;<BR>
import DBException;<BR>
<BR>
/**<BR>
 * Class to allow access to the database library<BR>
 */<BR>
public class Database<BR>
{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;// Table name to use (ODBC data source)
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public String tableName;<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;/**<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * Lone constructor.&nbsp;&nbsp;A data
source must be passed.<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * @param s holds the name of the data
source to use<BR>
&nbsp;&nbsp;&nbsp;&nbsp; */<BR>
&nbsp;&nbsp;&nbsp;&nbsp;Database(String s)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;tableName = s;
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;/**<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * Native method query<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * @param stmt holds the SQLStmt class
to use<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * @exception DBException is thrown on
any error<BR>
&nbsp;&nbsp;&nbsp;&nbsp; */<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public synchronized native void query(SQLStmt
stmt)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;throws DBException;
<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public synchronized native SQLStmt sql(String
stmt)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;throws DBException;
<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;static<BR>
&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.loadLibrary(&quot;Database&quot;);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
}</TT>
</BLOCKQUOTE>
<HR>
<P>
The first native method uses an SQLStmt object for both input
and output, and the second native method uses a String input and
returns an SQLStmt object as output. It is the native library's
task to create and fill the output object. Both native methods
are marked as synchronized because the library implementation
is single-threaded. Nothing in Java precludes making re-entrant
native libraries, but the database library uses global variables
for storage. This makes it necessary to protect the library from
being entered by more than one thread at a time.
<P>
As with the Demonstration class, the first step is to compile
the classes and pass them to the <TT>javah</TT>
tool:
<BLOCKQUOTE>
<TT>javac SQLStmt.java Database.java<BR>
javah SQLStmt Database</TT>
</BLOCKQUOTE>
<P>
Here is the output for the SQLStmt class:
<BLOCKQUOTE>
<TT>/* DO NOT EDIT THIS FILE - it is machine
generated */<BR>
#include &lt;native.h&gt;<BR>
/* Header for class SQLStmt */<BR>
<BR>
#ifndef _Included_SQLStmt<BR>
#define _Included_SQLStmt<BR>
struct Hjava_lang_String;<BR>
<BR>
typedef struct ClassSQLStmt {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;struct Hjava_lang_String *sqlStmt;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;struct HArrayOfArray *result;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;long nRows;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;long nCols;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;/*boolean*/ long query;<BR>
} ClassSQLStmt;<BR>
HandleTo(SQLStmt);<BR>
<BR>
#ifdef __cplusplus<BR>
extern &quot;C&quot; {<BR>
#endif<BR>
#ifdef __cplusplus<BR>
}<BR>
#endif<BR>
#endif</TT>
</BLOCKQUOTE>
<P>
Notice how the two-dimensional array has been translated into
HArrayOfArray. SQLStmt doesn't have any native methods, though
the <TT>javah</TT> tool still places
the surrounding <TT>ifdef cplusplus</TT>
statements where native methods would normally appear.
<P>
Here is the output for the Database class:
<BLOCKQUOTE>
<TT>/* DO NOT EDIT THIS FILE - it is machine
generated */<BR>
#include &lt;native.h&gt;<BR>
/* Header for class Database */<BR>
<BR>
#ifndef _Included_Database<BR>
#define _Included_Database<BR>
struct Hjava_lang_String;<BR>
<BR>
typedef struct ClassDatabase {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;struct Hjava_lang_String *tableName;<BR>
} ClassDatabase;<BR>
HandleTo(Database);<BR>
<BR>
#ifdef __cplusplus<BR>
extern &quot;C&quot; {<BR>
#endif<BR>
struct HSQLStmt;<BR>
__declspec(dllexport) void Database_query(struct HDatabase *,struct
HSQLStmt *);<BR>
__declspec(dllexport) struct HSQLStmt *Database_sql(struct HDatabase
*,struct Hjava_lang_String *);<BR>
#ifdef __cplusplus<BR>
}<BR>
#endif<BR>
#endif</TT>
</BLOCKQUOTE>
<P>
The two native methods appear at the bottom of the header. Since
this file has native methods, it needs to have stub code generated
for it. The next step is to execute the <TT>javah</TT>
tool with the stubs option:
<BLOCKQUOTE>
<TT>javah -stubs Database<BR>
</TT>
</BLOCKQUOTE>
<P><CENTER><TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR VALIGN=TOP><TD><B>Note</B></TD></TR>
<TR VALIGN=TOP><TD>
<BLOCKQUOTE>
There is no rule about what the stub module must be called. You can use the <TT>-ofilename</TT> option to override <TT>javah</TT>'s default naming convention. This option is also useful for forcing the output from multiple classes into a single stubs file: <TT>javah -stubs -ostubs.c class1 class2 class3</TT>. This can be done as long as all the native methods for classes 1, 2, and 3 will appear in the same native library.
</BLOCKQUOTE>

</TD></TR>
</TABLE></CENTER>
<P>
<P>
Here is the stub module for the Database class:
<BLOCKQUOTE>
<TT>/* DO NOT EDIT THIS FILE - it is machine
generated */<BR>
#include &lt;StubPreamble.h&gt;<BR>
<BR>
/* Stubs for class Database */<BR>
/* SYMBOL: &quot;Database/query(LSQLStmt;)V&quot;, Java_Database_query_stub
*/<BR>
declspec(dllexport) stack_item *Java_Database_query_stub(stack_item
*_P_,<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
struct execenv *_EE_) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;extern void Database_query(void *,void
*);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;(void) Database_query(_P_[0].p,((_P_[1].p)));
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;return _P_;<BR>
}<BR>
/* SYMBOL: &quot;Database/sql(Ljava/lang/String;)LSQLStmt;&quot;,
Java_Database_sql_stub */<BR>
declspec(dllexport) stack_item *Java_Database_sql_stub(stack_item
*_P_,<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
struct execenv *_EE_) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;extern long Database_sql(void *,void *);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;_P_[0].i = Database_sql(_P_[0].p,((_P_[1].p)));
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;return _P_ + 1;<BR>
}</TT>
</BLOCKQUOTE>
<P>
All that's left to do is to write the implementation module for
the Database stub. In the interest of having a comprehensible
project layout, the name of this module will be DatabaseImpl.c.