GlueGen JavaCallback: 1st Working Draft: Changed 'JavaCallbackDef' config, added code generation incl. native to Java dispatch and resource management

Tested via Test4JavaCallback.java (using test2.[hc]).

Please read the GlueGen_Mapping.md as well as Test4JavaCallback.java .

+++

Some implementation details:

JavaConfiguration maps JavaCallbackDef to JavaCallback set-function and maintains a list.
JavaCallbackDef itself holds all configured details.

JavaConfiguration also maps JavaCallbackInfo to JavaCallback set-function.
JavaCallbackInfo itself holds all compile time information, as produced by JavaEmitter.beginFunctions(..).
This extends JavaCallbackDef and avoid repetetive computation for the callback-function-type and its MethodBinding,
parameter indices for the callback interface and userParam, etc.

CMethodBindingEmitter: Native callback to Java dispatch
- The JavaCallback setter function creates a native 'UserParam' struct instance,
  which holds the callback-interface-jobject, its callback-jmethodID and
  the userParam-jobject for invocation of the actual JavaCallback interface method.

- To produce the C-Type -> JNI-Type conversion, An internal CMethodBindingEmitter instance
  for the native-callback function binding is created inside the CMethodBindingEmitter of the callback setter method.

  It is being used to map the types to JNI within the generated native callback function,
  passed to the actual JavaCallback method.

JavaMethodBindingEmitter: Native callback to Java dispatch
- The JavaCallbacl setter passes the callback-interface-object, the userParam-object and the
  callback-method-signature (to have the native method retrieve the jmethodID).

- It receives the native pointer of the native `UserParam` struct instance,
  which gets mapped to the userParam-object. (*TODO: Refine ownership + release*).

2 files changed, 253 insertions, 62 deletions

diff --git a/doc/GlueGen_Mapping.html b/doc/GlueGen_Mapping.html
index 9739082..02ec43f 100644
--- a/doc/GlueGen_Mapping.html
+++ b/doc/GlueGen_Mapping.html
@@ -426,11 +426,18 @@
   <li><a href="#alignment-for-compound-data">Alignment for Compound
   Data</a></li>
   </ul></li>
+  <li><a href="#oo-style-api-interface-mapping">OO-Style API Interface
+  Mapping</a>
+  <ul>
+  <li><a href="#oo-style-mapping-settings">OO-Style Mapping
+  Settings</a></li>
+  <li><a href="#oo-style-example">OO-Style Example</a></li>
+  </ul></li>
   <li><a href="#struct-mapping">Struct Mapping</a>
   <ul>
+  <li><a href="#struct-mapping-notes">Struct Mapping Notes</a></li>
   <li><a href="#gluegen-struct-settings">GlueGen Struct
   Settings</a></li>
-  <li><a href="#struct-mapping-notes">Struct Mapping Notes</a></li>
   <li><a href="#struct-setter-pseudo-code">Struct Setter
   Pseudo-Code</a></li>
   <li><a href="#struct-java-signature-table">Struct Java Signature
@@ -441,8 +448,18 @@
   Support</a></li>
   <li><a href="#struct-function-pointer-support">Struct Function-Pointer
   Support</a></li>
+  </ul></li>
   <li><a href="#java-callback-from-native-c-api-support">Java Callback
-  from Native C-API Support</a></li>
+  from Native C-API Support</a>
+  <ul>
+  <li><a href="#javacallback-constraints">JavaCallback
+  Constraints</a></li>
+  </ul></li>
+  <li><a href="#misc-configurations">Misc Configurations</a>
+  <ul>
+  <li><a
+  href="#libraryonload-librarybasename-for-jni_onload-"><code>LibraryOnLoad &lt;LibraryBasename&gt;</code>
+  for <code>JNI_OnLoad*(..)</code> ...</a></li>
   </ul></li>
   <li><a href="#platform-header-files">Platform Header Files</a></li>
   <li><a href="#pre-defined-macros">Pre-Defined Macros</a></li>
@@ -483,11 +500,30 @@ capable of representing all C types to represent the APIs for which it
 generates interfaces. It has the ability to perform significant
 transformations on the IR before glue code emission.</p>
 <p>GlueGen can produce native foreign function bindings to Java as well
-as map native data structures to be fully accessible from Java including
-potential calls to embedded function pointer.</p>
-<p>GlueGen is also capable to bind even low-level APIs such as the Java
-Native Interface (JNI) and the AWT Native Interface (JAWT) back up to
-the Java programming language.</p>
+as <a href="#struct-mapping">map native data structures</a> to be fully
+accessible from Java including potential calls to <a
+href="#struct-function-pointer-support">embedded function
+pointer</a>.</p>
+<p>GlueGen supports <a
+href="#java-callback-from-native-c-api-support">registering Java
+callback methods</a> to receive asynchronous and off-thread native
+toolkit events, where a generated native callback function dispatches
+the events to Java.</p>
+<p>GlueGen also supports <a
+href="#oo-style-api-interface-mapping">producing an OO-Style API
+mapping</a> like <a href="../../jogl/doc/uml/html/index.html">JOGL's
+incremental OpenGL Profile API levels</a>.</p>
+<p>GlueGen is capable to bind low-level APIs such as the Java Native
+Interface (JNI) and the AWT Native Interface (JAWT) back up to the Java
+programming language.</p>
+<p>Further, GlueGen supports generating <code>JNI_OnLoad(..)</code> for
+dynamic and <code>JNI_OnLoad_&lt;LibraryBasename&gt;(..)</code> for
+static libraries via <a
+href="#libraryonload-librarybasename-for-jni_onload-"><code>LibraryOnLoad Bindingtest2</code></a>,
+which also provides <code>JVMUtil_GetJNIEnv(..)</code> to resolve the
+<code>JNIEnv*</code> as used by <a
+href="#java-callback-from-native-c-api-support">Java callback
+methods</a>.</p>
 <p>GlueGen utilizes <a
 href="https://jogamp.org/cgit/jcpp.git/about/">JCPP</a>, migrated C
 preprocessor written in Java.</p>
@@ -930,6 +966,68 @@ Window(mingw/mingw64)</h4>
 +armv7l-eabi<br />
 - MacOsX-32bit-gcc4<br />
 ∗ Windows</p>
+<h2 id="oo-style-api-interface-mapping">OO-Style API Interface
+Mapping</h2>
+<p>GlueGen supports producing an OO-Style API mapping like <a
+href="../../jogl/doc/uml/html/index.html">JOGL's incremental OpenGL
+Profile API levels</a>.</p>
+<h3 id="oo-style-mapping-settings">OO-Style Mapping Settings</h3>
+<ul>
+<li><p><code>ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GL.java</code></p>
+<p>Ignore all extended interface symbols from named Java source
+file.</p>
+<p>The named Java source file is parsed and a list of its symbols
+extracted, allowing GlueGen to ignore these in the generated interface
+(here GLES3).</p>
+<p>This complements <code>Extends</code> setting, see below.</p></li>
+<li><p><code>Extends GLES3 GLES2</code></p>
+<p>The generated interface GLES3 extends interface GLES2.</p>
+<p>This complements <code>ExtendedInterfaceSymbolsIgnore</code> setting,
+see above.</p></li>
+<li><p><code>Implements GLES3Impl GLES3</code></p>
+<p>The generated implementation GLES3Impl implements interface
+GLES3.</p></li>
+</ul>
+<h3 id="oo-style-example">OO-Style Example</h3>
+<p>Example snippet from JOGL's GLES3 interface config
+<code>gl-if-es3.cfg</code></p>
+<pre><code>...
+
+ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GL.java
+ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GL2ES2.java
+ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GLES2.java
+ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GL2ES3.java
+ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GL3ES3.java
+ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GL4ES3.java
+ExtendedInterfaceSymbolsIgnore ../src/jogl/classes/com/jogamp/opengl/GLBase.java
+
+Package com.jogamp.opengl
+Style InterfaceOnly
+JavaClass GLES3
+Extends GLES3 GLES2
+Extends GLES3 GL4ES3
+...</code></pre>
+<p>Example snippet from JOGL's GLES3Impl implementation
+<code>gl-es3-impl.cfg</code></p>
+<pre><code>...
+ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GL.java
+ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GL2ES2.java
+ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GLES2.java
+ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GL2ES3.java
+ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GL3ES3.java
+ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GL4ES3.java
+ExtendedInterfaceSymbolsIgnore ../build-temp/gensrc/classes/com/jogamp/opengl/GLES3.java
+ExtendedInterfaceSymbolsIgnore ../src/jogl/classes/com/jogamp/opengl/GLBase.java
+
+Style ImplOnly
+ImplPackage jogamp.opengl.es3
+ImplJavaClass GLES3Impl
+Implements GLES3Impl GLES2
+Implements GLES3Impl GLES3
+...</code></pre>
+<p>Above produces the GLES3 interface and its implementation as visible
+in JOGL's UML document <a
+href="../../jogl/doc/uml/html/index.html">about OpenGL Profiles</a>.</p>
 <h2 id="struct-mapping">Struct Mapping</h2>
 <p>A <em>Struct</em> is a C compound type declaration, which can be
 mapped to a Java class.</p>
@@ -1826,43 +1924,104 @@ StructPackage T2_InitializeOptions com.jogamp.gluegen.test.junit.generation</cod
   
   /** Interface to C language function: &lt;br&gt; &lt;code&gt;int32_t CustomFuncB1(T2_UserData *  pUserData)&lt;/code&gt;&lt;br&gt;   */
   public final int CustomFuncB1(T2_UserData pUserData)  { .. }  </code></pre>
-<h3 id="java-callback-from-native-c-api-support">Java Callback from
-Native C-API Support</h3>
-<p>GlueGen supports registering Java callback methods to native C-API
-functions in the form:</p>
-<pre><code>typedef int32_t ( * T_CallbackFunc)(size_t id, const char* msg, void* userParam);
-
-void AddMessageCallback(T_CallbackFunc func, void* userParam);
-void RemoveMessageCallback(T_CallbackFunc func, void* userParam);
-void InjectMessageCallback(size_t id, const char* msg);</code></pre>
+<h2 id="java-callback-from-native-c-api-support">Java Callback from
+Native C-API Support</h2>
+<p>GlueGen supports registering Java callback methods to receive
+asynchronous and off-thread native toolkit events, where a generated
+native callback function dispatches the events to Java.</p>
+<p>C-API header snippet:</p>
+<pre><code>typedef void ( * T2_CallbackFunc01)(size_t id, const char* msg, void* usrParam);
+
+/** Sets the given `cbFunc` and associated `usrParam` as the callback. Passing NULL for `func` _and_ same `usrParam` removes the callback and its associated resources. */
+void MessageCallback01(T2_CallbackFunc01 cbFunc, void* usrParam);
+
+void InjectMessageCallback01(size_t id, const char* msg);</code></pre>
 <p>and the following GlueGen configuration</p>
-<pre><code>ArgumentIsString T2_CallbackFunc 1
-ArgumentIsString InjectMessageCallback 1
-
-# Define a JavaCallback, enacted on a function-pointer argument `T2_CallbackFunc` and a user-param `void*` for Java Object mapping
-JavaCallbackDef  T2_CallbackFunc 2</code></pre>
+<pre><code># JavaCallback requires `JNI_OnLoad*(..)` and `JVMUtil_GetJNIEnv(..)`
+LibraryOnLoad Bindingtest2
+    
+ArgumentIsString T2_CallbackFunc01 1
+ArgumentIsString InjectMessageCallback01 1
+        
+# Define a JavaCallback.
+#   Set JavaCallback via function `MessageCallback01` if `T2_CallbackFunc01` argument is non-null, otherwise removes the callback and associated resources.    
+#   It uses `usrParam` as the resource-key to map to the hidden native-usrParam object,    
+#   hence a matching &#39;usrParam&#39; must be passed for setting and removal of the callback.
+#
+#   It uses the function-pointer argument `T2_CallbackFunc01` as the callback function type
+#   and marks `T2_CallbackFunc01`s 3rd argument (index 2) as the mandatory user-param for Java Object mapping.
+#
+#   Note: An explicit `isMessageCallback01Mapped(Object usrParam)` is being created to explicitly query whether `usrParam` maps to the associated resources.
+JavaCallbackDef  MessageCallback01 T2_CallbackFunc01 2</code></pre>
+<p>Note that <a
+href="#libraryonload-librarybasename-for-jni_onload-"><code>LibraryOnLoad Bindingtest2</code></a>
+must be specified in exactly one native code-unit. It provides code to
+allow the generated native callback-function to attach the current
+thread to the <code>JavaVM*</code> generating a new
+<code>JNIEnv*</code>in daemon mode - or just to retrieve the thread's
+<code>JNIEnv*</code>, if already attached to the
+<code>JavaVM*</code>.</p>
 <p>This will lead to the following result</p>
 <pre><code>public interface Bindingtest2 {
 
-  /** JavaCallback interface: T2_CallbackFunc -&gt; int32_t (*T2_CallbackFunc)(size_t id, const char *  msg, void *  userParam) */
-  public static interface T2_CallbackFunc {
-    /** Interface to C language function: &lt;br&gt; &lt;code&gt;int32_t callback(size_t id, const char *  msg, void *  userParam)&lt;/code&gt;&lt;br&gt;Alias for: &lt;code&gt;T2_CallbackFunc&lt;/code&gt;     */
-    public int callback(long id, String msg, Object userParam);
+  /** JavaCallback interface: T2_CallbackFunc01 -&gt; void (*T2_CallbackFunc01)(size_t id, const char *  msg, void *  usrParam) */
+  public static interface T2_CallbackFunc01 {
+    /** Interface to C language function: &lt;br&gt; &lt;code&gt;void callback(size_t id, const char *  msg, void *  usrParam)&lt;/code&gt;&lt;br&gt;Alias for: &lt;code&gt;T2_CallbackFunc01&lt;/code&gt;     */
+    public void callback(long id, String msg, Object usrParam);
   }
 
   ...
+  
+  /** Entry point (through function pointer) to C language function: &lt;br&gt; &lt;code&gt;void MessageCallback01(T2_CallbackFunc01 cbFunc, void *  usrParam)&lt;/code&gt;&lt;br&gt;   */
+  public void MessageCallback01(T2_CallbackFunc01 cbFunc, Object usrParam);
+  
+  public boolean isMessageCallback01Mapped(final Object usrParam);
 
-  /** Entry point (through function pointer) to C language function: &lt;br&gt; &lt;code&gt;void AddMessageCallback(int32_t (*func)(size_t id, const char *  msg, void *  userParam), void *  userParam)&lt;/code&gt;&lt;br&gt;   */
-  public void AddMessageCallback(T2_CallbackFunc func, Object userParam);
-
-  /** Entry point (through function pointer) to C language function: &lt;br&gt; &lt;code&gt;void RemoveMessageCallback(int32_t (*func)(size_t id, const char *  msg, void *  userParam), void *  userParam)&lt;/code&gt;&lt;br&gt;   */
-  public void RemoveMessageCallback(T2_CallbackFunc func, Object userParam);
-
-  /** Entry point (through function pointer) to C language function: &lt;br&gt; &lt;code&gt;void InjectMessageCallback(size_t id, const char *  msg)&lt;/code&gt;&lt;br&gt;   */
-  public void InjectMessageCallback(long id, String msg);
-</code></pre>
-<p><em>TODO: Work in progress</em></p>
-<h4 id="example-1">Example</h4>
+  /** Entry point (through function pointer) to C language function: &lt;br&gt; &lt;code&gt;void InjectMessageCallback01(size_t id, const char *  msg)&lt;/code&gt;&lt;br&gt;   */
+  public void InjectMessageCallback01(long id, String msg);</code></pre>
+<h3 id="javacallback-constraints">JavaCallback Constraints</h3>
+<p>Please consider the following <em>currently enabled</em> constraints
+using <code>JavaCallbackDef</code></p>
+<ul>
+<li>Only one interface callback-method binding is allowed for a native
+callback function, e.g. <code>T2_CallbackFunc01</code> (see above).</li>
+<li>The native callback function can only return no-value, i.e.
+<code>void</code>, or a primitive type. Usually <code>void</code> is
+being used in toolkit APIs.</li>
+<li>The native callback function argument types must be able to be
+mapped to JNI Java types as supported for return values of all native
+functions, the same code path is being used within
+<code>CMethodBindingEmitter.emitBodyMapCToJNIType(..)</code>.</li>
+<li>To remove a JavaCallback the specified and mapped setter function,
+e.g. <code>MessageCallback01</code>, must be called with
+<code>null</code> for the callback interface but the very same
+<code>userParam</code> instance as previously called to set the
+callback.</li>
+<li>Exactly one native code-unit for the library must specify <a
+href="#libraryonload-librarybasename-for-jni_onload-"><code>LibraryOnLoad libraryBasename</code></a></li>
+<li>...</li>
+</ul>
+<p><em>TODO: Enhance documentation</em></p>
+<h2 id="misc-configurations">Misc Configurations</h2>
+<h3
+id="libraryonload-librarybasename-for-jni_onload-"><code>LibraryOnLoad &lt;LibraryBasename&gt;</code>
+for <code>JNI_OnLoad*(..)</code> ...</h3>
+<p><code>LibraryOnLoad &lt;LibraryBasename&gt;</code> generates native
+JNI code <code>JNI_OnLoad(..)</code> used for dynamic libraries,
+<code>JNI_OnLoad_&lt;LibraryBasename&gt;(..)</code> used for static
+libraries, <code>JVMUtil_GetJNIEnv(..)</code> and the instance of
+<code>JavaVM* &lt;LibraryBasename&gt;_jvmHandle</code>.</p>
+<p>The <code>JNI_OnLoad*(..)</code> methods set the
+<code>JavaVM* &lt;LibraryBasename&gt;_jvmHandle</code>, which in turn is
+utilized by <code>JVMUtil_GetJNIEnv(..)</code> to attach a new thread to
+the <code>JavaVM*</code> generating a new <code>JNIEnv*</code>in daemon
+mode - or just to retrieve the thread's <code>JNIEnv*</code>, if already
+attached to the <code>JavaVM*</code>.</p>
+<p>The <code>LibraryBasename</code> parameter is used to generate the
+<code>JNI_OnLoad_&lt;LibraryBasename&gt;(..)</code> variant for
+statically linked libraries. <code>JNI_OnLoad(..)</code>,
+<code>JNI_OnLoad_&lt;LibraryBasename&gt;(..)</code> and
+<code>JVMUtil_GetJNIEnv(..)</code></p>
 <h2 id="platform-header-files">Platform Header Files</h2>
 <p>GlueGen provides convenient platform headers,<br />
 which can be included in your C header files for native compilation and
diff --git a/doc/GlueGen_Mapping.md b/doc/GlueGen_Mapping.md
index ee23845..738c529 100644
--- a/doc/GlueGen_Mapping.md
+++ b/doc/GlueGen_Mapping.md
@@ -31,6 +31,10 @@ GlueGen can produce native foreign function bindings to Java as well as
 [map native data structures](#struct-mapping) to be fully accessible from Java including 
 potential calls to [embedded function pointer](#struct-function-pointer-support).
 
+GlueGen supports [registering Java callback methods](#java-callback-from-native-c-api-support)
+to receive asynchronous and off-thread native toolkit events,
+where a generated native callback function dispatches the events to Java.
+
 GlueGen also supports [producing an OO-Style API mapping](#oo-style-api-interface-mapping) like [JOGL's incremental OpenGL Profile API levels](../../jogl/doc/uml/html/index.html).
 
 GlueGen is capable to bind low-level APIs such as the Java Native Interface (JNI) and
@@ -752,49 +756,77 @@ and similar to `T2_CustomFuncB customFuncB1`
   public final int CustomFuncB1(T2_UserData pUserData)  { .. }  
 ```
 
-### Java Callback from Native C-API Support
-GlueGen supports registering Java callback methods to native C-API functions in the form:
-```
-typedef int32_t ( * T_CallbackFunc)(size_t id, const char* msg, void* userParam);
+## Java Callback from Native C-API Support
+GlueGen supports registering Java callback methods
+to receive asynchronous and off-thread native toolkit events,
+where a generated native callback function dispatches the events to Java.
 
-void AddMessageCallback(T_CallbackFunc func, void* userParam);
-void RemoveMessageCallback(T_CallbackFunc func, void* userParam);
-void InjectMessageCallback(size_t id, const char* msg);
+C-API header snippet:
 ```
+typedef void ( * T2_CallbackFunc01)(size_t id, const char* msg, void* usrParam);
 
-and the following GlueGen configuration
+/** Sets the given `cbFunc` and associated `usrParam` as the callback. Passing NULL for `func` _and_ same `usrParam` removes the callback and its associated resources. */
+void MessageCallback01(T2_CallbackFunc01 cbFunc, void* usrParam);
+
+void InjectMessageCallback01(size_t id, const char* msg);
 ```
-ArgumentIsString T2_CallbackFunc 1
-ArgumentIsString InjectMessageCallback 1
 
-# Define a JavaCallback, enacted on a function-pointer argument `T2_CallbackFunc` and a user-param `void*` for Java Object mapping
-JavaCallbackDef  T2_CallbackFunc 2
+and the following GlueGen configuration
 ```
+# JavaCallback requires `JNI_OnLoad*(..)` and `JVMUtil_GetJNIEnv(..)`
+LibraryOnLoad Bindingtest2
+    
+ArgumentIsString T2_CallbackFunc01 1
+ArgumentIsString InjectMessageCallback01 1
+        
+# Define a JavaCallback.
+#   Set JavaCallback via function `MessageCallback01` if `T2_CallbackFunc01` argument is non-null, otherwise removes the callback and associated resources.    
+#   It uses `usrParam` as the resource-key to map to the hidden native-usrParam object,    
+#   hence a matching 'usrParam' must be passed for setting and removal of the callback.
+#
+#   It uses the function-pointer argument `T2_CallbackFunc01` as the callback function type
+#   and marks `T2_CallbackFunc01`s 3rd argument (index 2) as the mandatory user-param for Java Object mapping.
+#
+#   Note: An explicit `isMessageCallback01Mapped(Object usrParam)` is being created to explicitly query whether `usrParam` maps to the associated resources.
+JavaCallbackDef  MessageCallback01 T2_CallbackFunc01 2
+```
+Note that [`LibraryOnLoad Bindingtest2`](#libraryonload-librarybasename-for-jni_onload-) must be specified in exactly one native code-unit.
+It provides code to allow the generated native callback-function to attach the current thread to the `JavaVM*` generating a new `JNIEnv*`in daemon mode -
+or just to retrieve the thread's `JNIEnv*`, if already attached to the `JavaVM*`.
 
 This will lead to the following result
 ```
 public interface Bindingtest2 {
 
-  /** JavaCallback interface: T2_CallbackFunc -> int32_t (*T2_CallbackFunc)(size_t id, const char *  msg, void *  userParam) */
-  public static interface T2_CallbackFunc {
-    /** Interface to C language function: <br> <code>int32_t callback(size_t id, const char *  msg, void *  userParam)</code><br>Alias for: <code>T2_CallbackFunc</code>     */
-    public int callback(long id, String msg, Object userParam);
+  /** JavaCallback interface: T2_CallbackFunc01 -> void (*T2_CallbackFunc01)(size_t id, const char *  msg, void *  usrParam) */
+  public static interface T2_CallbackFunc01 {
+    /** Interface to C language function: <br> <code>void callback(size_t id, const char *  msg, void *  usrParam)</code><br>Alias for: <code>T2_CallbackFunc01</code>     */
+    public void callback(long id, String msg, Object usrParam);
   }
 
   ...
+  
+  /** Entry point (through function pointer) to C language function: <br> <code>void MessageCallback01(T2_CallbackFunc01 cbFunc, void *  usrParam)</code><br>   */
+  public void MessageCallback01(T2_CallbackFunc01 cbFunc, Object usrParam);
+  
+  public boolean isMessageCallback01Mapped(final Object usrParam);
 
-  /** Entry point (through function pointer) to C language function: <br> <code>void AddMessageCallback(int32_t (*func)(size_t id, const char *  msg, void *  userParam), void *  userParam)</code><br>   */
-  public void AddMessageCallback(T2_CallbackFunc func, Object userParam);
-
-  /** Entry point (through function pointer) to C language function: <br> <code>void RemoveMessageCallback(int32_t (*func)(size_t id, const char *  msg, void *  userParam), void *  userParam)</code><br>   */
-  public void RemoveMessageCallback(T2_CallbackFunc func, Object userParam);
-
-  /** Entry point (through function pointer) to C language function: <br> <code>void InjectMessageCallback(size_t id, const char *  msg)</code><br>   */
-  public void InjectMessageCallback(long id, String msg);
-
+  /** Entry point (through function pointer) to C language function: <br> <code>void InjectMessageCallback01(size_t id, const char *  msg)</code><br>   */
+  public void InjectMessageCallback01(long id, String msg);
 ```
 
-*TODO: Work in progress*
+### JavaCallback Constraints
+Please consider the following *currently enabled* constraints using `JavaCallbackDef`
+- Only one interface callback-method binding is allowed for a native callback function, e.g. `T2_CallbackFunc01` (see above).
+- The native callback function can only return no-value, i.e. `void`, or a primitive type. Usually `void` is being used in toolkit APIs.
+- The native callback function argument types must be able to be mapped to JNI Java types as supported for return values of all native functions,
+  the same code path is being used within `CMethodBindingEmitter.emitBodyMapCToJNIType(..)`.
+- To remove a JavaCallback the specified and mapped setter function, e.g. `MessageCallback01`, must be called with `null` for the callback interface
+  but the very same `userParam` instance as previously called to set the callback.
+- Exactly one native code-unit for the library must specify [`LibraryOnLoad libraryBasename`](#libraryonload-librarybasename-for-jni_onload-)
+- ... 
+
+*TODO: Enhance documentation*
 
 ## Misc Configurations