add GnuPGData.size() to improve ByteArrayOutputStream performance

Since ByteArrayOutputStreams are used to get data out of GnuPGData, add a
size() method so that the ByteArrayOutputStream can pre-allocate the memory
needed, or at least close to the final number.

jni/GnuPGData.c

@@ -5,9 +5,17 @@
 #include "gpgmeutils.h"
 
 #include <gpgme.h>
+#include <data.h>
 
 #define BUFSIZE 1024
 
+
+JNIEXPORT jsize JNICALL
+Java_com_freiheit_gnupg_GnuPGData_gpgmeSize(JNIEnv * env, jobject self, jlong data)
+{
+    return (jsize) (DATA(data))->data.mem.size;
+}
+
 JNIEXPORT jlong JNICALL
 Java_com_freiheit_gnupg_GnuPGData_gpgmeDataNewFromMem(JNIEnv * env,
 						      jobject self,

src/com/freiheit/gnupg/GnuPGData.java

@@ -99,11 +99,11 @@ public void write(OutputStream out) throws IOException{
     /**
        Helper method to print out the data/string from this data object.
 
-       @return String representation of the data contained in this data object (expect weired results with binary data)
+       @return String representation of the data contained in this data object (expect weird results with binary data)
      */
     public String toString(){
         String result = null;
-        ByteArrayOutputStream baos = new ByteArrayOutputStream();
+        ByteArrayOutputStream baos = new ByteArrayOutputStream(this.size());
         try{
             this.write(baos);
             result = baos.toString();
@@ -135,6 +135,11 @@ protected void finalize(){
         destroy();
     }
 
+    public int size() {
+        return gpgmeSize(getInternalRepresentation());
+    }
+
+    private native int gpgmeSize(long l);
     private native long gpgmeDataNewFromMem(byte[] plain);
     private native long gpgmeDataNew();
     private native void gpgmeDataWrite(long l, OutputStream out) throws IOException;