Building Java Applications for the iSeries Server with VisualAge for Java 3.5

464 Building Java Applications for the iSeries Server with VisualAge for Java 3.5

11.4 Java Native Interface and C

This section discusses using the Java Native Interface and C on the iSeries server.

11.4.1 Setting up JNI and C

No special prerequisite source members are required to use JNI and C. The C and JNI for C

header files, which are actually source members, are included as part of OS/400 and the C

Licensed Program Product. These source members are copied into a C program when using

JNI. The source files are available on the iSeries server in the files shown in Table 11-1 and

Table 11-2.

Table 11-1 Prior to V5R1 source members and their locations on the iSeries server

In V5R1, the source files are in H file in the QSYSINC library.

Table 11-2 V5R1 source members and their locations on the iSeries server

Once the source members are located, follow these steps to use JNI and C:

1. Create a source physical file in your own library to store your own Java program header file

members. For any user-written Java program that is to be linked with C, you must create a

header file and store it in the source physical file. The command to create the header

source physical file is shown in Figure 11-46.

Parameter passing: There are problems with parameters passed by value of types jfloat,

jshort, jchar, and jbyte that are caused by parameter widening that is done by Java and C,

but not by RPG. We recommend that you

do not use float and short. Instead, replace them

with double and int.

Dealing with char and byte is trickier. It is best to avoid them, or pass them from or to Java

as an unsigned int. The problems do not always show up since the translator passes

primitives in registers whenever possible. Using a register for the parameter hides the

problem. Unfortunately, the problem shows up when the call statement becomes complex

enough to use up all of the registers. Therefore, some of the primitive parameters have to

be picked up from the stack.

Source member Location on the iSeries server

jni.h QSYSINC/H/JNI

QJAVA/H/JNI QJAVA/H/JNI

stdio.h QCLE/H/STDIO

stdarg.h QCLE/H/STDARG

jni_md.h QSYSINC/H/JNI_MD

QJAVA/H/JNI_MD QJAVA/H/JNI_MD

Source member Location on the iSeries server

jni.h QSYSINC/H/JNI

stdio.h QSYSINC/H/STDIO

stdarg.h QSYSINC/H/STDARG

jni_md.h QSYSINC/H/JNI_MD

Chapter 11. Java Native Interface (JNI) 465

Figure 11-46 Create Source Physical File (CRTSRCPF)

2. Generate a header file for each Java class that is to use JNI. Make sure that a class file

exists before you create your header file. For example, to create a JNI header file for the

HelloJ program, enter this command:

javah -jni HelloJ

The Java class files and header files are stored in an iSeries integrated file system

directory.

3. Convert the header file from the integrated file system into an iSeries library as a member

of the source physical file H. The previous step shows how to create the H file in your own

library. Figure 11-47 shows the command to copy and convert from the integrated file

system to a library file member.

Figure 11-47 Copy From Stream File (CPYFRMSTMF)

Figure 11-48 on page 466 shows an example of how the header file looks for the HelloJ Java

program.

Create Source Physical File (CRTSRCPF)

Type choices, press Enter.

File . . . . . . . . . . . . . . H Name

Library . . . . . . . . . . . APILIB Name, *CURLIB

Record length . . . . . . . . . 92 Number

Member, if desired . . . . . . . *NONE Name, *NONE, *FILE

Text 'description' . . . . . . . Header File for User JNI Headers

Copy From Stream File (CPYFRMSTMF)

Type choices, press Enter.

From stream file . . . . . . . . > '/home/a999501a/HelloJ.h'

To file member or save file . . > '/qsys.lib/apilib.lib/h.file/HelloJ.mbr'

Member option . . . . . . . . . > *ADD *NONE, *ADD, *REPLACE

Data conversion options . . . . *AUTO *AUTO, *TBL, *NONE

Stream file code page . . . . . *STMF 1-32767, *STMF, *PCASCII

Database file CCSID . . . . . . *FILE 1-65533, *FILE

End of line characters . . . . . *ALL *ALL, *CRLF, *LF, *CR...

Tab character expansion . . . . *YES *YES, *NO