# JNIWrapper Tutorial Version: 3.1 Last Updated: April 04, 2006 Copyright © 2002-2006 TeamDev Ltd. # Introduction Welcome to JNIWrapperTM for Microsoft Windows tutorial. It is designed to illustrate the creation of a simple application with certain features not provided by the Java platform and to demonstrate the basic concepts you need to know to develop successful applications using JNIWrapper. The program we are going to create will reveal JNIWrapper's capabilities through simple Win32 API functions. The first thing we'll do is to make some sounds using the system speaker. It's not very often that you hear Java programs are capable of such functionality, so this is the simplest way to make our application stand out. We'll call the program "Buzzer" to reflect the sound effect it produces and also add other nice features while going through the tutorial. The code will evolve as new concepts are introduced. By following the tutorial step by step and adding code, you will finally have a complete working program. The full Buzzer sample code is available for download on the Documentation page of the JNIWrapper site. # Chapter 1. Setting Up ## 1.1. Creating the Environment First things first. Let's set up the program working environment. Create a directory for the program and call it Sample. We will store our Java file, JNIWrapper library and any other resources here. It will also be our working directory for the sample application. In the working directory, let's create a directory for native files and call it `bin`. Now copy the Java library `jniwrap-3.1.jar` and `winpack-3.0.jar` to the Sample directory, the native DLL (`jniwrap.dll`) and license files (`jniwrap.lic`) to the `Sample\bin` directory. Create a Java application file in the `Sample` directory and call it `Buzzer`. You should now have the following structure:
        Sample\
               bin\
                   jniwrap.dll
                   jniwrap.lic
               Buzzer.java
               jniwrap-3.1.jar
               winpack-3.0.jar
## 1.2. Creating the Application Now let's set up the class of the application and run it before moving to the actual JNIWrapper coding. Put the following code into `Buzzer.java`:
        import com.jniwrapper.*;

        public class Buzzer
        {
            public Buzzer()
            {
            }
            public static void main(String[] args)
            {
                System.out.println("The Buzzer is running");
                Buzzer buzzer = new Buzzer();
            }
        }
Compile the class:
        javac -classpath jniwrap-3.1.jar;winpack-3.0.jar Buzzer.java
and run the application:
        java -classpath jniwrap-3.1.jar;winpack-3.0;. Buzzer
In this tutorial, compiling and running of the application should be understood as two actions being performed. Remember that you need to run the commands from the `Sample` directory for the program to work. OK, let's get to real work. # Chapter 2. Working with Native Libraries As was mentioned earlier, the first thing we'll try in our program is to make sounds using the system speaker. To do this, we'll use the standard Win32 `Beep` function. This function is exported from `kernel32.dll`, so our application requires this to be loaded. JNIWrapper provides a shortcut method for calling a function that loads a library and invokes its method. But the library itself provides a lot of interesting functions, so we'd better load it once to get access to the full variety of functions that we may choose to invoke. ## 2.1. Preparing a Search Path Before doing any library loading, we need to prepare a search path for our libraries, namely JNIWrapper DLL and those we'll use for invoking functions. By default, libraries are located by the `DefaultLibraryLoader` singleton instance. This class searches for libraries following the path specified by the `java.library.path` system property; though, using its `addPath` method, we can add new directories there. All standard Win32 libraries that we will use are on the system PATH, which is automatically added to `java.library.path`. So we just need to add a search path for the JNIWrapper DLL. (We could copy it to some location on the system PATH, but it is generally considered not a good style, especially for deployed applications.) The DLL is in the `bin` folder and since we run only from the current directory, let's use the relative path to it. We'll use the relative paths later to keep the example simpler. So, in our main function, we'll add a line as shown below:
    DefaultLibraryLoader.getInstance().addPath("bin");
You'll need to configure the library loader in all programs that use JNIWrapper before any native-related activity takes place. ## 2.2. Loading Native Code Libraries Let's get back to our library. To load a library, you should create an object of the `com.jniwrapper.Library` type specifying the library name. Since `kernel32.dll` will be used often in our program, we'll create a field for it and load it to the constructor. Now we need to change our class in the following way (the new code is given in bold):
        private Library _kernel;

        public Buzzer()
        {
            _kernel = new Library("kernel32");
        }
There is no need to specify the extension: it is appended automatically. Libraries are loaded when their objects are constructed. You don't have to worry about freeing libraries: if a library object is no longer used, the native library defined by it is unloaded. Now let's compile and run the application to make sure everything is typed correctly and all the libraries are available. # Chapter 3. Using Simple Types In this chapter, we'll make beeping sounds by invoking a native function with simple parameters. Let's put our "beeping" code in a new method called `buzz`. As was mentioned earlier, we'll use the `Beep` function to produce sounds. This function takes two `DWORD` parameters and returns a `BOOL` value. We are not really interested in the return value and will ignore it, since this function is unlikely to fail. However, we need to pass values, so we'll create objects for the function parameters:
        private void buzz()
        {
            // Prepare beep parameters: low and high frequencies and beep durations
                UInt32 low = new UInt32(400);
                UInt32 high = new UInt32(1000);
                UInt32 durLow = new UInt32(200);
                UInt32 durHigh = new UInt32(200);
We used the standard JNIWrapper's `UInt32` type which is exactly what `DWORD` is: a 32-bit unsigned integer. Our method will produce a series of lower and higher frequency beeps, that's why we define two sets of parameters: [`low; durLow`] and [`high; durHigh`]. In JNIWrapper, all numeric types, like `Int`, `SingleFloat`, and so on, have constructors that specify the initial value and also have getters and setters for the value. All parameters in JNIWrapper are mutable: they behave like variables. We could use one pair of parameters and modify their values between calls, but it's less illustrative, so let's have two sets. # Chapter 4. Invoking Functions Now that we have the library and parameters, one thing is still missing: the function to invoke. Functions are obtained from the library that exports them. To get a function, you should use the name it is exported with. Luckily, most Win32 API functions are exported with the same names (or almost the same - we will get to this case later) with which they appear in the documentation. To get the `Beep` function, let's use the following code:
            // Obtain function reference from the library
            Function beep = _kernel.getFunction("Beep");
Everything is ready to make beeping sounds. We'll create a small loop in which the `Beep` function is invoked to make high and low frequency beeps five times.
            // Do the beeping
            for (int i = 0; i < 5; i++)
            {
            beep.invoke(null, high, durHigh);
            beep.invoke(null, low, durLow);
            }
Note that the first argument of the `invoke` method is `null`. This is to indicate that we do not care about the return value. Later, we can easily add checking for it. Imagine the consequences of incorrectly ignoring the return value in the conventional JNI implementation: to get it later, you would need to modify the native method signature, change the native implementation and Java usages, and rebuild both Java and native code. This is just a simple example of how JNIWrapper can significantly save you development time. Our new method is complete. Let's invoke it from the main method to see how it works:
        public static void main(String[] args)
        {
            // Add JNIWrap.dll directory to library loader search path
            DefaultLibraryLoader.getInstance().addPath("bin");
            Buzzer buzzer = new Buzzer();
            buzzer.buzz();
        }
Compile and run the application. As a result, you should hear an alarm-like sound from the speaker. # Chapter 5. Using Strings Our next goal is to allow the user to run only one instance of the application at a time. Many Java programs try to achieve this by binding a particular TCP/IP socket. This approach does not ensure success because you cannot be absolutely sure the selected socket number is good. Our program will use a different method of creating a named mutex on start and checking for its existence to find out if another instance is running. With a well-chosen name, this method is practically bullet-proof. To create a mutex, we need to specify its name, which is a string. ## 5.1. String Types In Java, we have only one type of strings: the Unicode ones. Native side, however, has both single-byte (`char*`) and wide (`wchar_t*`) strings. JNIWrapper supports both types providing two concrete parameter classes: `AnsiString` for single-byte strings and `WideString` for wide strings. All strings we create in this application will consist only of 7-bit pure ASCII characters, so we'll use `AnsiString`s. To make sure that our single-instance mechanism is working properly, we need to make our program keep running for as long as needed. One of the simplest ways to achieve this is to display a message box at the stage where we would like to wait. Of course, we could use JOptionPane here, but it's not in the spirit of this tutorial and besides, it does not produce that cool native notification sound. Let's prepare a method that would allow displaying information or error message box:
        private void showMessageBox(String message, int flags)
The first argument here is the message itself, whereas the second one is the flags mask for the `MessageBox` function we are going to invoke. Let's take a closer look at the `MessageBox` function. From the documentation, we can see that it has the following signature:
        int MessageBox(
            HWND hWnd,          // handle to the owner window
            LPCTSTR lpText,     // text in the message box
            LPCTSTR lpCaption,  // message box title
            UINT uType          // message box style
        );
## 5.2. Creating Mutex and Displaying a Message We need to create a correct parameter set to invoke this function. The simplest way would be the `UINT` type as it has an equivalent type in JNIWrapper, i.e. `UInt`. As far as `HWND` is concerned, it is a handle and, therefore, a pointer. Generally speaking, pointers are more complex than the things we would like to touch upon in this chapter. Luckily, the data this pointer is pointing to will never be used, so we don't need to fill in this pointer's referenced area with any data. And last, but not least. In our case this pointer is simply `NULL`. On our platform, we could use a `UInt32` pararameter since we know that all pointers are unsigned 32-bit integers, but a more correct way would be to use a `Pointer.Void` parameter. We'll set its handle value to zero. This corresponds to a C-like conversion from an integer. Whenever you need a constant such as `(HWND)-1`, use `Pointer.Void` specifying the value in the constructor or setter. The rest of the parameters are strings. The `LPCTSTR` type defines a pointer to a string that is ANSI or Unicode, depending on the build configuration. Behind the scenes, it means that there are two versions for this function for each of the string formats. In Win32 API their names are created from a function name by appending 'A' for the ANSI version or 'W' for Unicode. We would like to use ANSI, so our target function is spelled `MessageBoxA`. The arguments are string pointers, so in order to pass them, we create instances of `AnsiString`. Strings in C/C++ are pointers to string data. In JNIWrapper, string parameters actually represent string data which, to become string arguments, need to be pointed to. But when we are calling a function, its string parameters are automagically passed to the underlying function as a pointer to relieve programmers from the burden of creating any extra pointers. Therefore, the string parameters for the `MessageBoxA` function are defined just like `AnsiString`. Here's the argument preparation code for our method:
            Pointer.Void hWnd = new Pointer.Void(0);
            AnsiString text = new AnsiString(message);
            AnsiString caption = new AnsiString("Buzzer");
            UInt uFlags = new UInt(flags);
To invoke the `MessageBoxA` function, we need another library - `user32.dll`. Add the following code in bold to the declarations and constructor:
        private Library _kernel;
        private Library _user;
        public Buzzer()
        {
            _kernel = new Library("kernel32");
            _user = new Library("user32");
            }
Let's finish the `showMessage` method:
            Function messageBox = _user.getFunction("MessageBoxA");
            messageBox.invoke(null, hWnd, text, caption, uFlags);
Here, we also can safely ignore the return value. For further convenience, we'll create methods to display the error and information message boxes. The methods will specify the required style constants for those message box types.
        private void message(String message)
        {
            showMessageBox(message, 0x30);
        }
        private void error(String message)
        {
            showMessageBox(message, 0x10);
        }
## 5.3. Passing Mutex Name Parameter With what we have already learned, it is really easy to write a mutex-based single instance checking part. First, let's create a parameter representing a mutex name:
        private boolean checkOneInstance()
        {
            AnsiString mutexName = new AnsiString("com.jniwrapper.sample.BuzzerMutex");
Then, let's try to open a mutex with this name:
            UInt32 desiredAcces = new UInt32(0x1F0001);
            Bool inheritHandle = new Bool(false);
            Pointer.Void mutexHandle = new Pointer.Void();
            Function openMutex = _kernel.getFunction("OpenMutexA");
            openMutex.invoke(mutexHandle, desiredAcces, inheritHandle, mutexName);
Here, we are interested in the result. If there is no such mutex, the function returns `NULL`, otherwise we'll know that another instance is running and we need to close the new instance. To get the return value, we need to create a variable of the required type and pass it as the first argument to the `invoke` method. After the invocation is complete, the passed parameter contains the return value. Next, we need to test the return value:
            if (!mutexHandle.isNull())
            {
                // Mutex exists - one instance is already running
                return false;
            }
## 5.4. Creating Locking Mutex Now if the mutex doesn't exist, this instance is the first one and therefore we need to create a locking mutex. Creating is not much different from checking, except for the part where the `NULL` result now indicates the error we would like to catch. Here is the rest of the checking method:
            // Not yet running - lock by creating mutex
            Pointer.Void mutexAttributes = new Pointer.Void(0);
            Bool initialOwner = new Bool(false);
            Function createMutex = _kernel.getFunction("CreateMutexA");
            createMutex.invoke(mutexHandle, mutexAttributes, initialOwner, mutexName);
            if (mutexHandle.isNull())
            {
                throw new RuntimeException("Mutex creation failed,
    last error = " + getLastError(true));
            }
            return true;
        }
## 5.5. Error Handling You have probably noticed that the last piece of code references a method that is not yet defined: `getLastError`. It needs to return the last system error code, optionally clearing the error status depending on a boolean parameter. Let's implement it now. We already know all methods and types to write the following simple piece of code:
        private long getLastError(boolean clear)
        {
            UInt32 r = new UInt32();
            Function getLE = _kernel.getFunction("GetLastError");
            getLE.invoke(r);
            long errCode = r.getValue();
            if (clear)
            {
                UInt32 zero = new UInt32(0);
                Function setLE = _kernel.getFunction("SetLastError");
                setLE.invoke(null, zero);
            }
            return errCode;
        }
## 5.6. Finishing Iteration We are ready to finish this iteration by adding the following code in bold to the `main` method:
        public static void main(String[] args)
        {
            // Add JNIWrap.dll directory to library loader search path
            DefaultLibraryLoader.getInstance().addPath("bin");
            Buzzer buzzer = new Buzzer();
            if (!buzzer.checkOneInstance())
            {
                buzzer.error("Buzzer is already running");
                System.exit(0);
            }
            buzzer.message("Buzzer started, click OK to close");
        }
Compile and run the application. As a result, you should see the "Buzzer started..." message box - don't close it and try to run another instance. Now you should see an error message box saying that the application is already running. # Chapter 6. Using Callbacks In this chapter, let's create a timer to make a sound after some time elapses. We'll use Windows native timers here, so we need a way to have the native code to call Java code. JNIWrapper provides such a capability using the `com.jniwrapper.Callback` class. You can create any number of callbacks that can have any parameters and return values. Creating a callback is simple: you need to subclass the `Callback` class, define the callback arguments and return value, and implement the `callback` method. Then, you pass an instance of this class as a parameter wherever you need a reference to the callback. ## 6.1. Creating a Timer Callback We are going to use the Windows API `SetTimer` function. It accepts the reference to the `TimerProc` callback function, so we need to implement such a callback. Here's its definition:
        VOID CALLBACK TimerProc(
          HWND hwnd,         // handle to window
          UINT uMsg,         // WM_TIMER message
          UINT_PTR idEvent,  // timer identifier
          DWORD dwTime       // current system time
        );
After examining the function signature, we can see we already know how to pass any of such types to a function call. Specifying them as callback arguments is no harder. Let's define our callback class as an inner class of our application class so that we can easily call the useful methods defined there:
        private class TimeOutCallback extends Callback
        {
            private Pointer.Void _hwnd = new Pointer.Void();
            private UInt _msg = new UInt();
            private UInt _timerID = new UInt();
            private UInt32 _time = new UInt32();
            public TimeOutCallback()
            {
We have defined the fields for each of our callback parameters. There is no return value, so we do not define any parameter for it. In the constructor, we need to configure our callback by specifying parameters of the callback signature:
                init(new Parameter[] {
                    _hwnd,
                    _msg,
                    _timerID,
                    _time
                }, null);
            }
The last `null` is the `void` return value. Now, to make this class complete and compilable, we'll implement the `callback` method. We'll make it just a buzz but will add more intelligent code to it later in this chapter:
            public void callback()
            {
                buzz();
            }
        }
The callback created is already usable. Easy, isn't it? Just imagine doing all this stuff using the conventional JNI techniques! ## 6.2. Using a Callback in the Application Now we need to create a timer and pass the callback reference to be called when the timer elapses. To do this, we'll create a method and call it startTimer. This function requires passing a window handle. We will use the `Wnd` class that can obtain a native window handle from any Swing component. In subsequent iterations, we'll get our own window handle without messing up with internal implementation classes. For now, let's create a Java window and use its handle. Add the following code in bold to the class initialization:
    import com.jniwrapper.*;

    import java.awt.*;
    import sun.awt.windows.WToolkit;
    import javax.swing.*;

    public class Buzzer
    {
        private Library _kernel;
        private Library _user;

        private Window _window;
        private static final int TIMER_ID = 1;

        public Buzzer()
        {
            _kernel = new Library("kernel32");
            _user = new Library("user32");
            _window = new JWindow();
            _window.setVisible(true);
        }
Please take a look at the new constant: it will be used in the code below. The window will just provide its handle to hook the timer to. Let's implement the `startTimer` method:
        private void startTimer(long timeout)
        {
            Wnd hWnd = new Wnd(_window)
            UInt eventID = new UInt(TIMER_ID);
            UInt timeOutVal = new UInt(timeout);
            UInt result = new UInt();
            TimeOutCallback timeOutCallback = new TimeOutCallback();
            Function setTimer = _user.getFunction("SetTimer");
            setTimer.invoke(result, hWnd, eventID, timeOutVal, timeOutCallback);
Passing a callback is even more straightforward than its implementation: just create an object and pass it to the function that requires it. It's just like an event listener. Some sanity check and we're done:
            if (result.getValue() == 0)
            {
                throw new RuntimeException("Failed to create a timer, error code = " + getLastError(true));
            }
        }
Before moving on, let's implement the `stopTimer` method to be called from the callback:
        private void stopTimer(Pointer.Void hwnd, UInt timerID)
        {
            Function killTimer = _user.getFunction("KillTimer");
            killTimer.invoke(null, hwnd, timerID);
        }
Here we used JNIWrapper types as parameters, because we already have them in the callback and there is no need to unwrap the values just to wrap them back. ## 6.3. Testing Results Let's test the resulting code. Modify the main method in the following way:
        public static void main(String[] args)
        {
            // Add jniwrap.dll directory to library loader search path
            DefaultLibraryLoader.getInstance().addPath("bin");
            Buzzer buzzer = new Buzzer();
            if (!buzzer.checkOneInstance())
            {
                buzzer.error("Buzzer is already running");
                System.exit(0);
            }
            buzzer.startTimer(5000);
        }
Compile the program. If it is already running, there is no control to terminate it. To close the program, just press `Ctrl+C` in its console (remember not to use `javaw.exe` here or else you'll have to use the Task Manager to stop the buzzing). Run the program. You should hear a regular beeping at 5-second intervals until the program is terminated. This means that the timer we are using is repetitive, so we'll need to stop it when it is no longer needed. ## 6.4. Improving Callback Code Now we'll complete the callback code with a few useful lines. We may want to: Stop the timer. Notify the user that the timer has elapsed. Finish the program. Here's the whole implementation of the `callback` method that does it all:
            public void callback()
            {
                stopTimer(_hwnd, _timerID);
                buzz();
                message("Timer has elapsed!");
                System.exit(0);
            }
Note that although our main thread terminates before the timer elapses, the program still lives because of non-daemon AWT threads that keep our window alive. Take a look at the first line of the function. When the callback is invoked, the variables specified at its creation are set to the argument values before the `callback` method is called. We pass two of the arguments to the `stopTimer` function. Our callback does not have a return value; if it had, we would need to assign the return value parameter before leaving the `callback` method. There are no limitations as to what things a callback can do other than those specified for the original callback. Compile and run the application. Five seconds after the start, the program will produce the usual sound, and then display the information message box saying that the timer has elapsed. Closing this message box will terminate the program. # Chapter 7. Using Structures In the previous chapter, we used an implementation-specific class to get the window handle. In this chapter, we are going to get one in a more legitimate way. We'll create a native window and later make it custom-shaped like a splash screen. Creating and managing a window using Windows API requires passing more complex parameters than we have used before, namely structures and pointers. We'll begin with structures. Any window has its event queue and requires some thread to dispatch its events. Messages are placed into the message queue in the form of the MSG structures. Let's examine the contents of this structure and implement it using JNIWrapper. Here is its definition:
    typedef struct tagMSG {
      HWND   hwnd;
      UINT   message;
      WPARAM wParam;
      LPARAM lParam;
      DWORD  time;
      POINT  pt;
    } MSG, *PMSG;
One of the MSG structure members is a structure itself: `POINT pt`. Its layout is as follows:
    typedef struct tagPOINT {
      LONG x;
      LONG y;
    } POINT, *PPOINT;
This one is really simple - it looks like a good starting point to learn how to create structures in JNIWrapper. Structures can be implemented by creating an instance of the `com.jniwrapper.Structure` class and initilazing them with objects representing fields. Most of them, however, are often reused and so it is usually better (and more readable) to create a subclass for each structure required. We'll choose the second approach:
        private static class POINT extends Structure
        {
            public LongInt _x = new LongInt();
            public LongInt _y = new LongInt();
First, we created a variable for each structure member. These variables are used for holding actual member data and for accessing it. Next, the structure must be initialized by defining its layout. Members are passed to the `init` method in the order they appear in the native-side structure declaration. Optionally, the structure alignment can be specified, but it is not needed here since all Windows API structures have the default alignment (of 1). Here is the rest of the code for this structure:
            public POINT()
            {
                init(new Parameter[] {_x, _y});
            }
        }
The `POINT` structure is ready. Using the same principle, let's create a class for the `MSG` structure:
        private static class MSG extends Structure
        {
            Pointer.Void _hwnd = new Pointer.Void();
            UInt _message = new UInt();
            UInt32 _wParam = new UInt32();
            UInt32 _lParam = new UInt32();
            UInt32 _time = new UInt32();
            POINT _pt = new POINT();

            public MSG()
            {
                init(new Parameter[]{_hwnd, _message, _wParam, _lParam, _time, _pt});
            }
        }
Note that defining a member of some complex type (structure) is no different from defining any member of simple ones. Uniformity is one of the JNIWrapper's main design goals. # Chapter 8. Using Pointers To demonstrate the use of a pointer in JNIWrapper, let's implement the event loop for our program. We'll do this before creating a window and doing all other preparation stuff, because using a pointer here is the simplest and most straightforward way. In fact, the only missing part will be the window handle, so let's assume that it is already stored in the `_hSplash` variable defined in the following way:
        private Pointer.Void _hSplash;
Add the above line to the declaration section of the class file. ## 8.1. Creating a Window Message Loop We'll implement the message loop in the `run` method of our program. The loop consists of sequential invocation of three API functions: `GetMessage`, `TranslateMessage` and `DispatchMessage`. However, instead of taking the MSG structure as their argument, they all require `LPMSG`, i.e. a pointer to that structure. In JNIWrapper, pointers are similar to any other type. To create a pointer, you need to create an instance of the `com.jniwrapper.Pointer` class. Each pointer should point to some other parameter called a referenced object. Whenever a pointer is written or read, its referenced object is also written or read, respectively. Let's create a pointer to the MSG structure:
        private void run()
        {
            Function getMessage = _user.getFunction("GetMessageA", null);
            Function translateMessage = _user.getFunction("TranslateMessage", null);
            Function dispatchMessage = _user.getFunction("DispatchMessageA", null);
            MSG msg = new MSG();
            Pointer msgPointer = new Pointer(msg);
The code related to the pointer creation is marked bold. In this example, `msgPointer` is a pointer and `msg` is its referenced object. In the previous code we used the `Pointer.Void` class instead of pointers, because we did not care for the referenced object and did not need to provide one. In this case, we need to provide the MSG structure so we need to use the real pointer. The rest of this function code uses the created pointer just as any other parameter:
            Bool result = new Bool();
            for (;;)
            {
                getMessage.invoke(result, msgPointer, _hSplash, new UInt32(0),
                                   new UInt32(0));
                if (!result.getValue())
                {
                    break;
                }
                translateMessage.invoke(null, msgPointer);
                dispatchMessage.invoke(null, msgPointer);
            }
        }
Although there is no need for us to access or modify data in the `msg` structure, we could easily do so. ## 8.2. Pointers and Strings Now that we learned all the techniques, we can go into the gory details of creating a window. First off, we will define the most complex structure in this example: the equivalent of Windows API WNDCLASS structure. We have already seen that any type can become a structure member equally easily. This structure will be another example: it will hold a callback reference. Add this class to your code:
        private static class WndClass extends Structure
        {
            private UInt32 _style = new UInt32();
            private Callback _lpfnWndProc;
            private Int32 _cbClsExtra = new Int32();
            private Int32 _cbWndExtra = new Int32();
            private Pointer.Void _hInstance = new Pointer.Void();
            private Pointer.Void _hIcon = new Pointer.Void();
            private Pointer.Void _hCursor = new Pointer.Void();
            private Pointer.Void _hbrBackground = new Pointer.Void();
            private AnsiString _lpszClassName = new AnsiString();

            public WndClass(Callback windowProc, String className,
                            Pointer.Void bgBrushHandle)
            {
                _style.setValue(3); // CS_HREDRAW | CS_VREDRAW
                _lpfnWndProc = windowProc;
                _lpszClassName.setValue(className);
                _cbClsExtra.setValue(0);
                _cbWndExtra.setValue(0);
                _hInstance.setValue(0);
                _hIcon.setValue(0);
                _hCursor.setValue(0);
                _hbrBackground.setValue(bgBrushHandle.getValue());
                init(new Parameter[] {_style, _lpfnWndProc, _cbClsExtra,
                     _cbWndExtra, _hInstance, _hIcon, _hCursor,
                     _hbrBackground, new Pointer.Void(0),
                     new Pointer(_lpszClassName)});
            }
        }
## 8.3. Using String Values in Structures Take a look at the initialization of the `_lpszClassName` field:
    init(new Parameter[] {... new Pointer(_lpszClassName)});
When defining structures that contain strings, you should always remember that there can be two types of them: pointers to character data and character arrays. To distinguish between the two, look at the member definition. Ones defined as `char *name` are pointers and others defined as `char name[20]` are arrays. In function calls, strings are always passed as pointers, so string parameters are automatically converted. But in structures, there is no way to know, so you should explicitly specify how the string is stored. If it is a pointer, a `Pointer` instance must be passed as the corresponding structure member. If it is an array, you should create a string with the maximum length equal to that of the expected character array and pass that parameter itself as the structure member. In this case, we have a pointer to characters version. # Chapter 9. Final Touch Now we are ready to finish the application. To do this, we need to implement a window procedure callback, register a window class, create a window, shape and center it, and define a window painting method. There is nothing special in any of these tasks so you can just take a look at the final code provided with this tutorial. After running it, you should see a small round window (on top of all other windows), in ten seconds the computer should make a beeping sound and pop up a message box saying that the timer has elapsed. Of course, the single instance rule is still enforced. The tutorial is over. By now you learned everything you need to start successfully using JNIWrapper in your applications.
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