How to call Java from C on Android ?

[Danny Backx]

Hi,

I need to be able to call Java from C code, and I have trouble getting
this to work in my sample program. Help :-)

Relevant code below. I tried with GetStaticMethodID +
CallStaticVoidMethod but that didn't work either (GetStaticMethodID
returns 0).

Also I'm not sure why this business with env vs lenv is happening. Code
was created by copying stuff from forum posts...

Danny

E/Yow ( 711): DoubleJniTest 1
E/Yow ( 711): DoubleJniTest NewStringUTF -> 0x43d114a0
E/Yow ( 711): DoubleJniTest GetObjectClass -> 0x43cffd00
E/Yow ( 711): DoubleJniTest GetStaticMethodID -> 0x412be468
I/DEBUG ( 27): *** *** *** *** *** *** *** *** *** *** *** *** ***
*** *** ***
I/DEBUG ( 27): Build fingerprint:
'generic/sdk/generic/:2.1/ERD79/22607:eng/test-keys'
I/DEBUG ( 27): pid: 711, tid: 711 >>>
net.sourceforge.projects.roadmap <<<
I/DEBUG ( 27): signal 11 (SIGSEGV), fault addr 00000004
I/DEBUG ( 27): r0 4104f100 r1 00000000 r2 40009408 r3 ad07ff50


<<RoadMap.java>>
package net.sourceforge.projects.roadmap;
...
public class RoadMap extends Activity
...
public void AddButton(String name) {
Button doit = new Button(thiz);

doit.setText(name);
buttons.addView(doit);
}

<< piece of jni/jni.c >>
static JavaVM *svm;
static jobject io;

jint
JNI_OnLoad(JavaVM *vm, void *reserved)
{
JNIEnv *env;
jclass cls;
jmethodID constr;

svm = vm;
if ((*vm)->GetEnv(vm, (void **)&env, JNI_VERSION_1_6) != JNI_OK)
return -1;
cls = (*env)->FindClass(env,
"net/sourceforge/projects/roadmap/RoadMap");
__android_log_print(ANDROID_LOG_ERROR, "JNI_OnLoad", "FindClass
-> %p", cls);
constr = (*env)->GetMethodID(env, cls, "<init>", "()V");
__android_log_print(ANDROID_LOG_ERROR, "JNI_OnLoad",
"GetMethodID -> %p", constr);
io = (*env)->NewGlobalRef(env, (*env)->NewObject(env, cls,
constr));
__android_log_print(ANDROID_LOG_ERROR, "JNI_OnLoad", "io -> %p",
io);

return JNI_VERSION_1_6;
}

void
JNI_OnUnload(JavaVM *vm, void *reserved)
{
JNIEnv *env;

(*vm)->GetEnv(vm, (void **)&env, JNI_VERSION_1_6);
(*env)->DeleteGlobalRef(env, io);
}

void
Java_net_sourceforge_projects_roadmap_RoadMap_DoubleJniTest(JNIEnv* env,
jobject thiz, char *s)
{
JNIEnv *lenv;
jclass c;
jmethodID m;
char * ns;

__android_log_print(ANDROID_LOG_ERROR, "Yow", "DoubleJniTest
1");

if ((*svm)->GetEnv(svm, (void **)&lenv, JNI_VERSION_1_6) < 0) {
__android_log_print(ANDROID_LOG_ERROR, "Yow",
"DoubleJniTest error");
return;
}

ns = (*lenv)->NewStringUTF(lenv, "Yow baby");
__android_log_print(ANDROID_LOG_ERROR, "Yow", "DoubleJniTest
NewStringUTF -> %p", ns);

c = (*lenv)->GetObjectClass(lenv, io);
__android_log_print(ANDROID_LOG_ERROR, "Yow", "DoubleJniTest
GetObjectClass -> %p", c);
m = (*env)->GetMethodID(env, c, "AddButton",
"(Ljava/lang/String;)V");
__android_log_print(ANDROID_LOG_ERROR, "Yow", "DoubleJniTest
GetStaticMethodID -> %p", m);
(*env)->CallVoidMethod(env, c, m, ns);

__android_log_print(ANDROID_LOG_ERROR, "Yow", "DoubleJniTest
4");

}


--
Danny Backx ; danny.backx - at - scarlet.be ; http://danny.backx.info

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[laurent mallet]

I recommand to look source code of kwaak. You will find many
interesting code.

http://code.google.com/p/kwaak3/

Laurent,

[fadden]

On May 20, 1:31 pm, Danny Backx <danny.ba...@scarlet.be> wrote:
> Relevant code below. I tried with GetStaticMethodID +
> CallStaticVoidMethod but that didn't work either (GetStaticMethodID
> returns 0).

Don't use GetStaticMethodID on a non-static method.

> void
> Java_net_sourceforge_projects_roadmap_RoadMap_DoubleJniTest(JNIEnv* env,
> jobject thiz, char *s)
> {

[...]

>         if ((*svm)->GetEnv(svm, (void **)&lenv, JNI_VERSION_1_6) < 0) {

You have an "env", passed in. You don't need to query for it.

>         c = (*lenv)->GetObjectClass(lenv, io);

[...]

>         (*env)->CallVoidMethod(env, c, m, ns);

The second argument is the instance of the object on which you are
calling the method. You're passing in a class object. This is not
going to work.

It looks like you want to CallVoidMethod(env, io, m, ns). Note "ns"
is a jstring, not a char*.

[Danny Backx]

Thanks for both replies, I got things to work now, see below.

Danny

void
Java_net_sourceforge_projects_roadmap_RoadMap_DoubleJniTest(JNIEnv* env,
jobject thiz, char *s)
{

jclass cls;
jmethodID mid;
int i;
jstring js;

cls = (*env)->FindClass(env,
"net/sourceforge/projects/roadmap/RoadMap");

if (cls == 0)
return;

mid = (*env)->GetMethodID(env, cls, "AddDummyButton", "(I)I");
if (mid == 0)
return;
i = (*env)->CallIntMethod(env, thiz, mid, 5);

js = (*env)->NewStringUTF(env, "yow");

mid = (*env)->GetMethodID(env, cls, "AddButton",
"(Ljava/lang/String;)V");
if (mid == 0)
return;
(*env)->CallVoidMethod(env, thiz, mid, js);

}


public void AddButton(String name) {
Button doit = new Button(thiz);
doit.setText(name);
buttons.addView(doit);
}

public int AddDummyButton(int a) {

Button doit = new Button(thiz);

buttons.addView(doit);
return 123;

}


--
Danny Backx ; danny.backx - at - scarlet.be ; http://danny.backx.info

[siva]

Hi,
Nice explanation, but still i've confusion. So I request you to send me the
complete code of RoadMap.java and jni.c.

Can you please post here??

[siva prasad reddy]

Hi Everybody!!

I need to be able to call Java from C code, and I have trouble getting

this to work in my sample program. I'm getting blank screen..Help :-)

Relevant code below.
RoadMap.java
--------------------------------------------------------------------------------------------

public class RoadMap extends Activity

{
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
//AddButton("Hello");
this.DoubleJniTest();

}

public void AddButton(String name) {
LinearLayout buttons = (LinearLayout) findViewById(R.id.container);
Button doit = new Button(this);
doit.setText(name);
buttons.addView(doit);
System.out.println("------------- Add buttons..");
}
/*public int AddDummyButton(int a) {
LinearLayout buttons = (LinearLayout) findViewById(R.id.container);
Button doit = new Button(this);
buttons.addView(doit);
return 123;
}*/
public native void DoubleJniTest();
static
{
System.loadLibrary("jni");
}
}
//----------------------------------------------------------------------------------------------------------------------------
jni.c

#include <jni.h>
#include <android/log.h>
#include "com_example_roadmap_RoadMap.h"

static JavaVM *svm;
static jobject io;
jint
JNI_OnLoad(JavaVM *vm, void *reserved)
{
JNIEnv *env;
jclass cls;
jmethodID constr;
svm = vm;
if ((*vm)->GetEnv(vm, (void **)&env, JNI_VERSION_1_6) !=
JNI_OK)
return -1;

cls = (*env)->FindClass(env, "com/examples/roadmap/RoadMap");
__android_log_print(ANDROID_LOG_ERROR, "JNI_OnLoad",
"FindClass-> %p", cls);

constr = (*env)->GetMethodID(env, cls, "<init>", "()V");
__android_log_print(ANDROID_LOG_ERROR, "JNI_OnLoad",
"GetMethodID -> %p", constr);
io = (*env)->NewGlobalRef(env, (*env)->NewObject(env, cls,
constr));
__android_log_print(ANDROID_LOG_ERROR, "JNI_OnLoad", "io ->
%p", io);
return JNI_VERSION_1_6;
}

void
JNI_OnUnload(JavaVM *vm, void *reserved)
{
JNIEnv *env;
(*vm)->GetEnv(vm, (void **)&env, JNI_VERSION_1_6);
(*env)->DeleteGlobalRef(env, io);
}
void

Java_com_examples_roadmap_RoadMap_DoubleJniTest(JNIEnv* env, jobject
thiz)

{
jclass cls;
jmethodID mid;
int i;
jstring js;

cls = (*env)->FindClass(env,"com/examples/roadmap/RoadMap");

if (cls == 0)
return;

/**mid = (*env)->GetMethodID(env, cls, "AddDummyButton",

"(I)I");
if (mid == 0)
return;

i = (*env)->CallIntMethod(env, thiz, mid, 5); */
js = (*env)->NewStringUTF(env, "Got It!");

mid = (*env)->GetMethodID(env, cls, "AddButton", "(Ljava/lang/
String;)V");
if (mid == 0)
return;
(*env)->CallVoidMethod(env, thiz, mid, js);
}

//-------------------------------------------------------------------------------------------------------------------------
main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/
android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:id="@+id/container"
>
<TextView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="@string/hello"
/>
</LinearLayout>

[Christopher Van Kirk]

Did you try the methods one might use to call from native into Java on any
other platform?

}

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[siva prasad reddy]

I've tried on Android Platform. Now i'm able to run.

Thanks for  your reply!!

Regards
-------------
Siva Prasad Reddy S,
Associate Software Engineer - Trainee,
ArijaSoft,
Hyderabad.

[Sarker Tanveer Ahmed Rumee]

Could you please help regarding how you did it Android?