Calling java.lang.Runtime methods from C - help

[Stephen Kellett]

Hi folks,

I've called Java methods in classes of my own creation from C without
trouble. However I have a job where I need to do the following from C.
Here is the Java that would do the job.

long value;

value = java.lang.Runtime.getRuntime().getFreeMemory();

I can get the class "java.lang.Runtime", however when I then try to get
the method id for the static method getRuntime() it fails (returns with
a 0 method id).
Same when I try to get the non-static method "getFreeMemory()" it fails.
Any ideas?

JNIEnv *Env // assume this is valid (in my code it is checked and
valid)

jlong value = 0;
jclass jc = NULL;
jmethodID jmid = NULL;

jc = Env->FindClass("java.lang.Runtime");
if (jc != NULL)
{
jobject runtime;

jmid = Env->GetStaticMethodID(jc,
"getRuntime",
"()Ljava.lang.Runtime;");
if (jmid != NULL)
{
runtime = Env->CallStaticObjectMethod(jc, jmid);
if (runtime != NULL)
{
jmid = Env->GetMethodID(jc,
methodName,
methodSignature);
if (jmid != NULL)
{
value = Env->CallLongMethod(runtime,
jmid);
}
}
}
}

Alternatively, if someone knows how to get the memory stats in C without
doing the above that would be cool.

Stephen

 

[Stefan Schulz]

"()Ljava.lang.Runtime;");

Slashes... ()Ljava/lang/Runtime; and you should be set.

 

[Gordon Beaton]

I can get the class "java.lang.Runtime", however when I then try to get
the method id for the static method getRuntime() it fails (returns with
a 0 method id).
Same when I try to get the non-static method "getFreeMemory()" it fails.
[...]

jc = Env->FindClass("java.lang.Runtime");
if (jc != NULL)
{
jobject runtime;

jmid = Env->GetStaticMethodID(jc,
"getRuntime",
"()Ljava.lang.Runtime;");

The most common reason for not finding method or field ids is an
incorrect signature. Use javap -s <classname> to check!

This is the correct way to express it:

"()Ljava/lang/Runtime;"

/gordon

 

[Stephen Kellett]

The most common reason for not finding method or field ids is an
incorrect signature. Use javap -s <classname> to check!

This is the correct way to express it:

"()Ljava/lang/Runtime;"

Doh! Thank you very much. Bangs head against wall.

Stephen