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Java Backend

Ehc
Note: this information is now included in the source tree in  text/InternalDocJazy.cltex, so this page is really obsolete.

---++ Java Backend

---+++ IO
To do IO, the UHC standard library relies on C code which is called through the Haskell FFI. To make IO work in the Java backend without rewriting large parts of the standard library, we have two options:
   * Use the Java Native Interface (JNI) to call the C code that's also used by the other backends. This is described below.
   * Re-implement the foreign functions necessary (but not the Haskell functions using them) in Java. This is what we decided to do in UHC (as of 2010/04/20).

---+++ Using JNI for IO
To do this, we need to do the following things:
   * globally:
      * Emulate pointers in Java.We can't use real C pointers because JNI does not support "pinning" memory, and pointers become invalid after a native method call return.
      * Compile the C code used to a dynamically-linked library.
      * Load the library at runtime, using System.load(..) or System.loadLibrary(..).
   * for each function:
      * Write a wrapper method in Java.
      * Declare a native method in Java.
      * Write a wrapper function in C that calls the C function doing the real work.

As an example, here's the code needed for the =write= function. This assumes that there is a class uu.jazy.ehc.NativeMethods containing the native method declarations.
   * in src/jazy/uu/jazy/ehc/Prim.java:<verbatim>
public static Object write(int fd, int buf, int count) {
    byte[] array = (byte[])pointers.dereference(buf);
    int result = nativeMethods.write(fd, array, count);
    return ioReturn(new Integer(result));
}
</verbatim>
   * in src/jazy/uu/jazy/ehc/NativeMethods.java:<verbatim>
public native int write(int fd, byte[] buf, int count);
</verbatim>
   * in C code:<verbatim>
JNIEXPORT jint JNICALL Java_uu_jazy_ehc_NativeMethods_write(
        JNIEnv *env, jobject obj, jint fd, jbyteArray buf, jint count) {
    jbyte *ptr = (*env)->GetPrimitiveArrayCritical(env, buf, 0);
    int result = write(fd, ptr, count);
    (*env)->ReleasePrimitiveArrayCritical(env, buf, ptr, 0);
    return result;
}
</verbatim>