acronym JNI (Java Native Interface). It comes as part of the JDK (Java Development Kit). How you write
C/ C++/assembler code native methods callable
from Java. It also lets C/C++ code call Java
methods. Microsoft J++ does not support JNI. Before you get too deeply into JNI
check out the exec function. Also check if JConfig has what you
need. It may do what you want with much less hassle. Also check out XFunction which lets you do JNI without writing any of that complicated
Tying Java and C
together is a messy
business. Unless you are constrained by something such as:
- legacy code without source
- proprietary specialty library
- platform-specific code
- device drivers
then you will likely be happiest converting everything to one language. In any case, you want to keep the
corpus callosum between the two languages as small as possible with as little traffic as possible. In other
words, when in Java
, attempt to stay in Java
as long as possible. When in C
, attempt to stay as long as possible in C
. Share as little data as possible back and forth.
Keep the formats of shared data as minimal as possible.
JNI is how
you write C/C++/assembler code native methods
callable from Java. It also lets C/ C++ code
call Java methods.
You can’t directly manipulate or create C++ objects
from Java. You need to write native method implementation code in C++ to allow
java objects and methods to indirectly create and manipulate the C++ objects.
Your C++ code has to do the C++ object
creating and manipulating based on clues passed from Java by native methods and parameters.
The Overall Process
- If you need to use JNI from an Applet, order a certificate well in advance.
See Signed Applet.
- Put all your classes in a package. Package-less classes are just for tiny toy programs.
- Write a Java class containing a native method something like this Glue.java:
- Compile Glue.java with javac.exe in the usual way. This
step is very important. You must have a clean compile before using javah.exe.
- Generate the Glue.h header file containing the prototypes of the C/C++ methods you must write with:
javah.exe -jni -o Glue.h com.mindprod.JNIExper.Glue
- Write a C or C++ class something like this Glue.c using the code in glue.h as a template.
- Link that C code and any assembler code it calls into a DLL (Dynamic Link Library).
The DLL may contain methods from many different classes.
- Pre-install the DLL on the java.library.path i.e. on the browser’s
classpath or path, e. g. J:\Program Files\java\jdk1.8.0_05
\jre\bin\ext (for the JDK 1.8.0
plug-in), WINNT\java\trustlib (for Internet Explorer), Program Files\Netscape\Communicator\java\classes (for Netscape 8.0).
You can’t change DLLs (Dynamic Link Libraries)
to a new version without rebooting, not even the
disk copy, let alone the RAM (Random Access Memory) image. Java can change code on the fly without so
much as stopping the program. Surely Microsoft could work out a method to change
a DLL that did not require such drastic measures. Some Unix machines run for
years without rebooting.
To replace a DLL with a new one, you
must use the inuse utility to allow you to replace the
DLL and then reboot to clear the in-RAM copy.
inuse test.dll C:\winnt\system32\test.dll /y
Alternatively, you can reboot, and then
replace the DLL, so long at it is not loaded.
- Alternatively, you can install the DLL to a temporary file in any random directory the browser uses
as the current directory. You have to install it afresh on every run. The problem is, because it is a
can’t delete it until the next reboot. File.delete() and File. deleteOnExit() will
fail. There is no such thing as File.deleteOnReboot(), so I suggest using HunkIO. createTempFile which generates filenames that will be more easily recognized as junk and
discarded later by some disk cleaner, or by your own program on a subsequent run.
- Use the Java native method as if it were an ordinary Java method.
- Normally, Java calls C/C++, but you can
do the reverse. Normally you just pass primitives back and forth. On the C++
side, you can create a Java object with the JNI API (Application Programming Interface) and
populate its fields and return it, most commonly a String. You can also create
C++ objects and use them on the C++ side.
They mean nothing on the Java side so you can’t bring them over into Java without converting them to Java
objects first. Similarly, you can’t create arbitrary C++ objects from
the Java side.
Some general JNI tips
- See if you can avoid JNI altogether. Let your Java talk to your native code via
TCP/IP (Transmission Control Protocol/Internet Protocol).
and web-loaded Applets are incompatible, even if you sign them or jump through incredible hoops. You must use a
Signed Applet to install a second signed one on the
client’s local hard disk, and then use that second Applet to do your JNI
work. Even then loadLibrary is unreliable.
- A JNI call is very slow, in the order of .5 to 1.0 microseconds, the equivalent of
pages of linear Java code to do a simple method call. You would think there would be a tiny generated machine
code thunk to bridge between Java and C. Not so — at least in any JVM (Java Virtual Machine).
I know of, Java branches to some general purpose code that
interpretively constructs the C parameters. This code is not highly optimised. It seems Sun
wants to strongly discourage you from using native methods just for speed. This means you don’t want to
hop back and forth between Java and C, but rather to go to C, and stay there a decently long time before
returning. This means that you can’t use C to speed up short operations, only long ones, because of the
overhead tacked on in getting to C wipes out any savings.
- Keep your JNI interface as minimal as possible. This is tricky, messy, kludgy stuff. Do as
little native code as possible. Confine it to as few classes as possible.
- Get your native code working first in an application before you tackle the complexities of signed Applets and capabilities and permissions. With applications you
can control the loadlibrary path with:
java -Djava.library.path=. HelloWorld.
With Applets, you are at the mercy of the browser.
- If you do your native code in C++, there is slightly more type checking.
Watch out, you use env-> in C++ rather than
(*env)-> you would use in C. Keep aware in C++ when
the first argument is hidden.
- When you compile, set the Options/Project/Directory include path to:
J:\Program Files\java\jdk1.8.0_05\include\win32, or equivalent. Note that JNIEXPORT
defined in win32\jni_md.h. For Borland, make sure target expert says:
Win32 console, static and Project/Options/Linker/General is no debug, otherwise you will generate enormous
- None of the Java safety net is operational when your native code is running. Keep in mind the tiniest error
in your JNI code will crash the JVM. In Java version 1.2 there is a debug mode that does extra
parameter checking invoked with -Xcheck:jni. Build your code incrementally, testing
thoroughly before adding a few more lines.
- Check the return status of every call. There is no exception mechanism to do it for you.
- Make sure you understand global and local references, and include the necessary DeleteLocalRef and DeleteGlobalRefs. When you return an Object, you only need
to pass back a local reference. If you want to retain a long-term reference to the Object for
yourself, that lives after your method returns, then you need to convert it to a global
reference. Local references are automatically deleted when your method returns. You must eventually delete
global references explicitly. You may optionally delete local references prematurely. Similarly, you must
explicitly use ReleaseStringUTFChars to free the C-style string created from a Java
Unicode string that you allocated with GetStringUTFChars. If you fail to release, your
code will run for a while, then eventually die of the memory leak. These can be a bear to track down. Code
carefully and deal with the release the instant you write the code for the allocate.
- Be careful with GetIntArrayRegion and GetIntArrayElements and modifying the C-array you get. Since GetIntArrayRegion gives you a copy, any changes you make will not affect the Java array.
GetIntArrayElements effectively gives you a direct pointer to the Java array so any
changes you make to the C array are reflected in the Java array. However, there is a slight catch.
GetIntArrayElements may be implemented two ways:
- Giving you a direct pointer to the Java array. In that case, any changes you make are instantly
reflected in the Java array. This technique is used when the garbage collector supports pinning —
freezing the Java array at a fixed location, even through a GC (Garbage Collection)
- Giving you a copy of the Java array. In that case, any changes you make are not reflected until you
call ReleaseIntArrayElements, when your C array is automatically copied back into
the Java array. This technique is used when the garbage collector does not support pinning and there would
be no way to freeze the Java array at a fixed location where C expects it.
- AllocObject does not invoke the constructors. It is up to you to
initialise all the fields. Normally you would use NewObject.
- When debugging DLL s, remember the client must sometimes have to reboot to force Windows or NT to
reload the new version of the DLL. DLL hell strikes again. Why must Microsoft fool around with dancing paperclips
instead of fixing this? One way around the problem is to keep changing the name of your
- When you are debugging, display the code and DLL version number on the screen so that you can detect if you
are inadvertently using an old version of your Applet stuck in cache, or an old version of a
- If you have a class file twice on the classpath you will get a misleading error: java.lang.ClassFormatError: Class already loaded. It has nothing to do with the
already being loaded. Thankfully, having a DLL still loaded from the last time you ran is not considered an
- It is also probably a good idea to give your class containing the native method, your C code, and the
- When you do your javah -jni com.mindprod.mypackage.MyClass, make sure you use
dots in the name, not slashes.
- When you do your javah -jni com.mindprod.mypackage.MyClass, make sure you have
freshly compiled it first. It works from the *.class file, not the *.java file.
- ListDLLs is a
useful tool to find out which NT DLLs
are currently loaded.
- Don’t use thread local storage (a Microsoft C++ feature) with any
that you let the JVM load. TLS (Transport Layer Security) doesn’t work if the library is
loaded with loadLibrary.
- I’m told you don’t have to buy an expensive C/ C++ compiler to do your JNI work. The Gnu/CygnWin shell and C++ compiler is available free
for many platforms.
- Though you can work in either C or C++, you always use extern 'C' so that C-style parameter pushing order is always used.
- Before you write your own JNI native class, check out JConfig. It may already be done.
- If you have a dual CPU (Central Processing Unit) s,
be even more tricky. It has not yet been properly tested in that configuration.
- If you are using a 64-bit JVM,
you will need to compile
64-bit C++ JNI.
If you are using a
32-bit JVM, you will need to compile 32-bit C++ JNI.
JNI Primitive Types To Use in C/C++
See also the table of sizeof native C++ types.
|JNI Primitive Types To Use in C/C++|
||typedef unsigned char jboolean;
||8-bit signed byte
||typedef signed char jbyte;
||16-bit unsigned short,
||typedef unsigned short WORD;
||16-bit unsigned char
||16-bit unsigned char, wide char
||typedef wchar_t WCHAR;
||16-bit unsigned char on Unicode supporting systems, 8-bit unsigned char
or older systems, text char
||typedef WCHAR TCHAR;
||16-bit unsigned char
||typedef unsigned short jchar;
||16-bit signed short
||typedef short jshort;
||32-bit unsigned int, double word
||typedef unsigned long DWORD;
||DWORD *, pointer to 32-bit unsigned int, long pointer double word
||typedef DWORD far * LPDWORD;
||32-bit Boolean. TRUE/true=1 FALSE/false=0, Boolean
||typedef int BOOL;
||32-bit signed int
||32-bit signed int
||32-bit signed int
||typedef long jint;
||32-bit signed float
||typedef float jfloat;
||64-bit signed long
||64-bit signed long
||typedef __int64 jlong;
||char *, pointer to 8-bit null-terminated string
||typedef __nullterminated CHAR * LPSTR;
||const char *, constant pointer to 8-bit null-terminated string, long
pointer constant string
||typedef __nullterminated CONST CHAR * LPCSTR;
||wchar_t *, pointer to 16-bit null-terminated string on
Unicode-supporting platforms. On older platforms it means char *, pointer to 8-bit null-terminated string, long pointer text string
||typedef LPCWSTR LPCTSTR;
||const wchar_t *, constant pointer to 16-bit null-terminated string on
Unicode-supporting platforms. On older platforms it means char *, pointer to 8-bit null-terminated string, long pointer constant text string
||typedef LPCWSTR LPCTSTR;
||64-bit signed double
||typedef double jdouble;
||counted Unicode String
||the JNI environment with hooks to the JNI
JNI Manipulator Functions
you opaque access to the Java objects. You never touch the Java objects directly, you always manipulate
them via rather clumsy remote access methods. It is bit like being a blind brain surgeon using barbecue tongs.
The advantage is your program never need know what the actual format of the objects is. It makes it much easier
to write portable C/C++ code.
JNI accessor functions
In the above table, wherever you see Int, you can replace it with Boolean, Byte, Char, Short, Long, Float or Double. Note these methods do not follow Java capitalisation conventions.
|Useful JNI functions to Access Parameters|
converted to 16-bit chars
||The result of GetStringChars is not null delimited! You must copy and append
your own null with wcsncpy_s.
C++ Unicode 16-bit
functions do not work (quietly degrade to 8-bit mode) unless you define
converted to 8-bit chars
The result of GetStringUTFChars is automatically null delimited, though Sun
documentation is unclear. You might wonder at the asymmetry with GetStringChars. 16-bit strings need not copy. GetStringChars can us the original java 16-bit string which
is not null terminated. 8-bit requires a copy, so GetStringUTFChars might as well append a null while it at it.
||use parm directly
||use local jint|
||use parm directly
|static field in Object
|instance field in Object
|callback static method
|callback instance method
Typical JNI C Code
Here is some typical JNI C code to open a file, that lets you access a Java string inside C.
Typical JNI C++ Code
Here is the C++ code for accessing the volume serial number of drive under
Compiling JNI C/C++ Code
Presuming your JNI class with native methods is called Mouse you
will need to write a C or C++ program called mouse.cpp that contains the methods for nativemouse.dll. Your C/C+
program mouse.cpp will implement the methods in the generated mouse.h file.
Get a clean compile of your Java code, then use javah.exe like this:
If you are using Microsoft’s commercial C/C++ compiler you must have J:\Program Files\java\jdk1.8.0_05
\include and J:\Program Files\java\jdk1.8.0_05
\include\win32 in the include list. To configure click: Tools | options | Projects and
Solutions | VC++ directories | include files or click: tools | options | directories |
For project as a whole, configure: project | settings | general | no
MFC (Microsoft Foundation Classes) and In project | settings | link | output filename |
should end in DLL
For the free MS Visual C++ Express 12, configure your project as a DLL
library. Include the two
libraries J:\Program Files\java\jdk1.8.0_05\include and J:\Program Files\java\jdk1.8.0_05
\include\win32 in the /I section when you first define the project, (not forced includes) or change them
later in right click | project properties| C/ C++ | additional include directories. Every time the JDK
changes, you must manually
change every JNI project file for both debug and release.
Turn off incremental linking. Turn off the 64-bit warnings.
In project | C++ | code generation set the
runtime library /MT option to statically link (i.e. include system run time code in the
than link to the runtime. Alternatively, you must install the C++ runtime on
all clients, available from:
The command line equivalents to your GUI (Graphic User Interface) options should look something like this
Use the batch rebuild to ensure both debug and release versions compile and link without
JNI and Assembler
To write the JNI code partly in assembler, there are two approaches:
Microsoft C conventions return an int value in eax or a
long in edx:eax. You can learn the register conventions by adding the /FA option to the project C++ settings and looking at the
generated *.ASM code for C++ or C programs.
- Use Microsoft Visual C++ inline assembler.
- Use a traditional external 32-bit assembler, and create a little C glue
Here is a sample using inline Assembler. Leaving a value in eax without a
ret is how you return a value, even though it generates a compiler warning message.
- Using assembler or inline assembler can massively shrink your DLL
files. So often in C, one tiny
function brings in all kinds of library code.
- If you work in assembler, you can get a rough prototype in ASM (Assembler)
to work with by writing a
dummy version in C, and getting the compiler to generate ASM
source. In Borland you do this
with Project ⇒ right click on *.c file ⇒ Special ⇒ C->ASM.
- Test your actual ASM code with a test C driver. Once you get that working, try putting it in a
After that is working, attempt calling it from Java.
- Beware! assemblers may only handle 8.3 source names, and may truncate public symbols or convert them to
upper case. It gets pretty strange. Java, C, C++ and
MASM (Microsoft Assembler) each have their own idea of what the symbol names are.
Unless you specify to the C++ compiler that symbols are extern 'C', it will decorate them with method signatures. Sometimes symbols get a leading
_. You can guess these symbols from MAPs, error messages, and looking at hex dumps of
object files. I found it easiest to call a C wrapper method with the official Java name which in turn calls an
ordinary assembler routine with a short name, all uppercase.
- In theory, with a smart linker, you should be able to alias the short name to the long one and avoid the C
wrapper method. With the wrapper method, you don’t need the JNIEXPORT on your
prototype, since the JNI interface never sees it, e. g. jint JNICALL NATIVE1(JNIEnv
*env, jclass thisClass, jint n);
- Make sure you are explicit about JNICALL on your MASM prototype which means the called routine is responsible for
popping the parms with a ret n.
- Borland inline assembler does not work in the C compiler in 32-bit mode
unless you buy the separate TASM product.
- You can’t access the familiar 16-bit DOS (Disk Operating System)
int 21h functions, from a 32-bit flat app.
- The method signature strings that GetMethodID wants can be had from running
javap -s -p on your compiled Java class files.
JNI Example Code
I have posted five programs with source that use JNI : FileTimes, Mouse,
Pentium, Volser, SetClock.
Using JNI in Applets
I drove myself nuts trying to get JNI to work with signed Applets back in the days of Netscape proprietary signing and
JVMs (Java Virtual Machines). I gave up and went with Java Web Start. It may be easier now. What following are
notes from those terribly frustrating times.
Using native methods in a Netscape Applet is a bear because even after you manage to defang the security
PrivilegeManager.enablePrivilege( "UniversalLinkAccess" );
to let you call your native methods,System.loadLibrary( mydll) and the
undocumented security system, will insist that the *.DLL file containing the your native
methods and all your classes be pre-installed on the client’s machine. It refuses to look for them in the
jar file or on the website where CODEBASE points. You are pretty well stuck making your Applet install the
and class files on the client’s machine. Ouch! System. loadLibrary fails for some reason if the DLL was not present at the time Netscape fired up. The
System. loadLibrary can’t seem to see a
installed dynamically. This makes no sense since the DLL is not loaded until System.
loadLibrary is called. Even more baffling is why System.
load would show the same behaviour. I have fooled around with this for months and I
still could not get Netscape System. loadLibrary to behave
reliably and predictably. Until that problem is solved, I consider it practically impossible to use
I got word from Hannu Helminen that it is possible after all. He came across this JavaWorld Article. What is says basically that
Netscape and native code do work together after all. He tried it out, and to his amazement the example indeed
works! But hey, he did the same things that I did, where is the catch?
The idea is that first you download the native DLL and a class file to user’s local hard drive. The class
file has to be in the Netscape class path, thus it uses the system class loader. Also the
DLL has to
be downloaded and System. loaded before
the class is referenced for the first time. It appears that Netscape does some kind of checking for the
already when the class is loaded. I have not yet had time to check this out myself.
To make it worse, there are fourteen security bypassing schemes you have to deal with.
System.load vs System.loadLibrary
takes a fully qualified filename, e.g. C:\dlls\myjni.dll, one ending in *.dll (or *.so for Linux or Solaris). System.
loadLibrary takes an unqualified filename, e.g. myjni and
appends the .dll or .so for you. The idea is you can write more
platform-independent code this way. I have had more success with System.load
during debugging then flipping to System.loadLibrary
once all is working. I suspect there is a mother of a gotcha hidden in loadLibrary
not yet revealed. Check out the system property java.library.path with Wassup to see where the restricted system property
is looking for DLLs. It will be the usual executable path plus a few extra directories. Alternatively
find out the library
System.out.println ( System.getProperty( "java.library.path" ) );
In an application or the Opera browser, you can determine the library load path with:
String lib = System.getProperty( "java.library.path" );
You must explicitly load the corresponding DLL
before using any class inside
it. On W98/Me/NT/W2K/XP/W2003/Vista/W2008/W7-32/W7-64/W8-32/W8-64, that library path is
supposed to contain:
The library path depends on whether you wrote an Applet or application, which browser
and the phases of the moon.
- The Windows system directories.
- The current working directory.
- The entries is the PATH (not CLASSPATH) set
- and sometimes, at least in Java Web Start, the root directory of your
must be added to jars without the package name path.
make sure you put your JNI shared object library *.dll somewhere on the library
path. If your use System.loadLibrary
( dog ), then you must name your library file with your
compiled C++ code dog.dll. Put dog.dll directly on the library path, not in a package name sprouting off the library path. If no
existing directory in the java.library.path is suitable, put the dll in some other
directory, and add that directory to the ordinary executable path by
adding it in the control panel set environment. It will be automatically included in java.library.path. To share a your native library on a LAN (Local Area Network),
assign a dummy network drive letter to the server:directory where you put it, and
add that to the path of each client. With JWS, you can put
dog.dll in a jar, but don’t give it a package name.
Solaris and Linux loadLibrary
In Linux, to compile and link the C/C++ code,
to tell it where to find the JNI headers. In Linux there is an environment variable called LD_LIBRARY_PATH that controls the path where *.so files are searched for.
On Solaris or Linux, make sure you put your JNI shared object library *.so somewhere
on the library path. If your use System. loadLibrary(
dog ), then you must name your library file with
your compiled C++ code libdog.so. Put libdog.so directly on the library path, not in a package name sprouting off the library path. With
JWS (Java Web Start), you an put libdog.so in a jar,
but don’t give it a package name.
On the Macintosh, make sure you put your JNI shared object library lib*.jnilib
somewhere on the library path. If your use System. loadLibrary(
dog ), then you must name your library file with
your compiled C++ code libdog.jnilib. Put
libdog.jnilib directly on the library path, not in a package name sprouting off the
library path. With JWS, you can put libdog.jnilib in a jar, but don’t
give it a package name. Apple Java has a Java-access to the proprietary Mac API,
so you don’t often need
DLL in the Jar
If you want to put your DLLs in the jar, you must bundle them as resources, then to load them, read the resource from
the jar and copy them to a temp file, then a System.load the temporary file.
The process is somewhat more complicated that you might think. See Notes on how.
File Naming Conventions
Naming things so that the various parts can find each other is perhaps the trickiest part of
does not matter exactly what naming convention you use, just that you be 100%
consistent. Here is the scheme I use in production. I put each class with JNI
into its own package.
file naming conventions for JNI
|File Naming Conventions for JNI|
||The main Java class that contains some native methods.|
||The C program that implements the native methods.|
||The C header file generated by Javah giving the C prototypes for the native methods to be implemented
||The DLL native executable library containing the native implementations. Before it
can be used, it must be copied somewhere on java.library.path, the system path, or in the jar
without a package name. You load the dll, using in a static init of the Mouse class, with System. loadLibrary ( nativemouse );Note
the lack of .dll or path information.|
C++ called via JNI knows nothing about Java exceptions. Java exceptions created
in C++ are just control blocks lying about. They have no automatic effect on
program flow. It is up to you in some C++ish way, after you set up the exception, to return quickly and
gracefully up the call stack to your caller back in Java who can then handle
the exception. On your way back, C++ can handle or notice the exception by
explicitly testing for it with ExceptionCheck.
Here are basics of how to make JNI threadsafe:
- The JNIEnv pointer is valid only for the thread associated with it. Don’t
pass it around.
- Local references are only valid in the thread that created them.
- You can convert a local reference to global reference to share it.
- You have the same sorts of synchronised problems in JNI
you do in regular Java, it is
just you deal with them with MonitorEnter and MonitorExit. For those who like to line dangerously, it is also possible to use the native
JNI methods useful in reflection
The equivalent JNI C code for the following fragment of Java is much more long winded:
|JNI Methods Useful In Reflection|
||returns superclass of a class reference.|
||checks whether instances of one class can be used when instances of another are expected.|
||return the class of a given jobject.|
||checks whether a jobject is an instance of a given class.|
||Convert a java.lang.reflect.Field
to a field ID.|
||Convert a to a field ID to a java.lang.reflect. Field.|
or a java.lang.reflect. Constructor to a method
||Convert method ID to a java.lang.reflect.Method
or a java.lang.reflect. Constructor.|
Class theClass = Class.forName( "java.lang.Long" );
Constructor constructorList = theClass.getConstructors();
The Old Netscape Problem
This describes a problem with earlier versions of Netscape. The new versions work quite differently. Netscape
won’t let web-loaded Applets invoke DLL code, even if they have UniversalLinkAccess permission. Further, it won’t let them use a custom ClassLoader to do it
indirectly. You may bypass this with the undocumented MarimbaInternalTarget class.
Your custom classloader must do a Class.getClass() first before attempting to fulfill the request itself.
The Old Netscape Solution
This applies to earlier versions of Netscape. New ones behave quite differently. You don’t need to deal
with any of this security, installing and jar-signing stuff if you use an application instead of an Applet. I
strongly suggest that approach wherever possible.
I have fooled around with this over a period of six months, chasing wild goose after wild goose, and have
finally came to the conclusion, in agreement with Sun’s FAQ (Frequently Asked Questions),
that JNI and Applets simply don’t mix. There is simply no way to get sufficient
security clearance to let you directly access the DLL from a web-loaded Applet, even if you write a custom
ClassLoader. One problem with doing so many tests is I could have slipped somewhere along the line, thinking I
tested two cases, when I actually tested only one. The problem is the way Windows/Netscape hold onto the old
code. I have not even got the method I describe below to work. It may fail too. Netscape security may apply even
if you load from local hard disk.
What you have to do is use a small signed installer Applet to install a second unsigned Worker Applet on the
client’s local hard disk. When that second Worker Applet runs, it is totally free of security restrictions,
and so can access JNI DLLs. It behaves much like an application, except it runs under a browser.
You also have to install some html in that same local directory that will load the Worker Applet from the
local hard disk. It would have CODE and ARCHIVE parameters, but no CODEBASE. It defaults to the local hard disk
directory where the html file lives.
You have to install the DLL in F:\Program Files\Netscape\Communicator\program\java\bin, which is guaranteed to be on
Netscape’s Windows path, where Windows looks for DLLs.
You have to install the unsigned Worker jar file in F:\Program Files\Netscape\Communicator\program\java\classes. Netscape totally trusts classes
it loads from the local file system, even if they are not signed and have no capabilities calls.
The Old Netscape Recipe
Just follow this recipe, if this discussion is making your brain hurt. The same technique will work for other
platforms with the obvious substitutions. If you do understand it, you can create your own shortcuts.
In order to execute JNI methods from a Netscape Applet, create three jar files.
- installer.jar. When this signed jar is first executed, it installs the various files on
the client’s local hard disk, (intelligently choosing C: or D:). On subsequent executions, it notices the
needed files are already installed and up-to-date and avoids that step. As soon as it has ensured it has
installed the files, it uses getAppletContext().showDocument(url) to transfer
control to the HTML (Hypertext Markup Language) it has
downloaded on C: or D: The installer jar contains only
the tools such as FileTransfer classes for help
in downloading and installing the files. It does not contain any of the data to be installed. Keeping that out
of installer.jar saves transferring that bulk when it is already installed.
- Worker.jar is for your class files that contain native methods, and the other classes you
need to run the actual Applet. This jar should not be signed. If you sign it, it will slow class loading the
code down. The Worker.jar will be embedded inside the toInstall jar, described shortly. The installer Applet
will copy the Worker.jar to F:\Program Files\Netscape\Communicator\program\java\classes or perhaps D:. Thereafter, you
could run it standalone via a bookmark, or you could run it via the original install.jar. The advantage of
using the original install.jar is automatic updates, and automatic finding where the Worker.jar Applet is
installed. The disadvantage is extra startup time and an extra annoying grant to run the program each
- toInstall.jar. This unsigned jar is just a container for the various files you need to
- An HTML file to invoke the actual Applet. Your installer Applet will install it in:
F:\Program Files\Netscape\Communicator\program\java\classes, or perhaps
- Your DLL file containing the native C++/C/Assembler
code. Your installer Applet will install it in F:\Program Files\Netscape\Communicator\program\java\bin, or perhaps D:.
- Your worker Applet jar containing all the class files you will need to run the unsigned Applet. Your
installer Applet will install it in F:\Program Files\Netscape\Communicator\program\java\classes or perhaps D:.
If you use getResourceAsStream, you must use the goofy extension *.ram for resources inside your jar files because Netscape interferes with the extensions
*.dll, *.exe, *.class etc. If you
access them via ZipFile that kludge is not necessary.
The key to debugging JNI is to write a C/C++ test harness (unit test) to test your C/C++ application code. Then when it is working write the JNI
glue to call your C/ C++ application methods. Let’s say, for example,
you were going to write some code to get at the CPU serial number like Pentium. You write a C method to get the serial number. You write a C/ C++ mainline that calls your method and prints out the
serial number on the console. You use pure C/ C++ debugging tools to get this all working. Then once you are sure your method is working,
you write the JNI glue to call it from Java. The JNI
is pretty mechanical after you
have done it a few times. You don’t do anything fancy in the JNI
code. Anything tricky you do in
the C/ C++ method or in the Java code that
calls the native method. JNI is ugly and so you want to keep it as simple as possible, as free of application
logic as possible.
Another strategy is to write dummy JNI methods in standard Java. Introduce your real
one class/method at a time and see when it starts to blow up. That localises the problem.
I see references to com.sun.jna.win32.StdCallLibrary,
which looks like a way to access the Windows API, but I can’t find any documentation on it.
In Windows, 32 and 64-bit DLLs
use the same extension *.dll so you must either provide two distributions, each either
pure 32-bit or pure-64-bit, on provide both versions in separate directories. Or you might name your
with a suffix of 32.dll, 86.dll or 64.dll. The contents of Program Files (x86)\Java\jdk1.7.0_51\include and
Program Files\Java\jdk1.7.0_51\include are the same, including the Win32 directory. I
gather it is purely the C++ compiler directives that decide whether to generate 32 or 64 bit code. I don’t
have a 64-bit C++ compiler, so I have never tried the experiment.
You likely are saying to yourself, What a production! There must be a simpler way.
Here are some alternatives:
- Spawn a C++ program with the exec
- Use a JNI library, or JNI generator written by someone else.
- Exchange data via data files, perhaps big or little-endian binary, or XML (extensible Markup Language).
- Exchange data via an SQL (Standard Query Language) database.
- Exchange data via a TCP/IP socket.
My essay has only scratched the surface. You must have a text book if you hope for any
success with JNI.
Book referral for The Java Native Interface, Programmer’s Guide and Specification
||recommend book⇒The Java Native Interface, Programmer’s Guide and Specification|
|Sun Microsystems. Does not cover Applet signing, or obvious JNI like accessing int parms, but he does explain many fine points well. A slim, indispensible, expensive book. The specification itself is bundled as part of the JDK docs. Part of the book is available free online.|
|Greyed out stores probably do not have the item in stock. Try looking for it with a bookfinder.|
Book referral for Essential JNI, Java Native Interface
||recommend book⇒Essential JNI, Java Native Interface|
||Rob Gordon [Author], Alan McClellan [Editor]
|This book is aimed more at the beginner than Liang’s book. It can’t very well teach you C and Java and JNI in one book, but it does not make quite so many assumptions about what you already know.|
|Greyed out stores probably do not have the item in stock. Try looking for it with a bookfinder.|
Oracle’s Technote Guide on JNI Specification
Sun appears to have withdrawn the Beth Stearns tutorial.