Trail: Deployment
Lesson: Doing More With Java Rich Internet Applications
Section: JNLP API

使用 JNLP API 访问客户端

When launched by using the Java Network Launch Protocol (JNLP), rich Internet applications (RIAs) can access the client with the user's permission. Consider the Text Editor applet example to understand how to use JNLP API based services. The Text Editor has a text area and buttons labeled Open, Save, and SaveAs. The Text Editor can be used to open an existing text file, edit it, and save it back to disk.

The Text Editor applet is shown next.

注意: If you don't see the applet running, you need to install at least the Java SE Development Kit (JDK) 6 update 10 release.

注意: If you don't see the example running, you might need to enable the JavaScript interpreter in your browser so that the Deployment Toolkit script can function properly.

The TextEditor and TextEditorApplet classes lay out the user interface and display it as an applet. The FileHandler class contains the core functionality with respect to using JNLP API based services.

Remember, the techniques described in this topic apply to Java Web Start applications as well.

To make use of a JNLP service, first retrieve a reference to the service. The initialize method of the FileHandler class retrieves references to JNLP services as shown in the following code snippet:

private static synchronized void initialize() {
    ...
    try {
        fos = (FileOpenService)
            ServiceManager.lookup("javax.jnlp.FileOpenService");
        fss = (FileSaveService)
            ServiceManager.lookup("javax.jnlp.FileSaveService");
    } catch (UnavailableServiceException e) {
        ...
    }
}

After you have a reference to the required services, invoke methods on the service to perform the necessary operations. The open method of the FileHandler class invokes the openFileDialog method of the FileOpenService class to display a file chooser. The open method returns the contents of the selected file.

public static String open() {
    initialize();
    try {
        fc = fos.openFileDialog(null, null);
        return readFromFile(fc);
    } catch (IOException ioe) {
        ioe.printStackTrace(System.out);
        return null;
    }
}

Similarly, the save and saveAs methods of the FileHandler class invoke corresponding methods of the FileSaveService class to enable the user to select a file name and save the contents of the text area to disk.

public static void saveAs(String txt) {
    initialize();
    try {
        if (fc == null) {
            // If not already saved.
            // Save-as is like save
            save(txt);
        } else {
            fc = fss.saveAsFileDialog(null, null,
                                         fc);
            save(txt);
        }
    } catch (IOException ioe) {
        ioe.printStackTrace(System.out);
    }
}

At runtime, when the RIA attempts to open or save a file, users see a security dialog asking them if they want to allow the action. The operation will proceed only if users allow the RIA to access their environment.

The complete source of the FileHandler class is shown next.


// add javaws.jar to the classpath during compilation 
import javax.jnlp.FileOpenService;
import javax.jnlp.FileSaveService;
import javax.jnlp.FileContents;
import javax.jnlp.ServiceManager;
import javax.jnlp.UnavailableServiceException;
import java.io.*;

public class FileHandler {

    static private FileOpenService fos = null;
    static private FileSaveService fss = null;
    static private FileContents fc = null;

    // retrieves a reference to the JNLP services
    private static synchronized void initialize() {
        if (fss != null) {
            return;
        }
        try {
            fos = (FileOpenService) ServiceManager.lookup("javax.jnlp.FileOpenService");
            fss = (FileSaveService) ServiceManager.lookup("javax.jnlp.FileSaveService");
        } catch (UnavailableServiceException e) {
            fos = null;
            fss = null;
        }
    }

    // displays open file dialog and reads selected file using FileOpenService
    public static String open() {
        initialize();
        try {
            fc = fos.openFileDialog(null, null);
            return readFromFile(fc);
        } catch (IOException ioe) {
            ioe.printStackTrace(System.out);
            return null;
        }
    }

    // displays saveFileDialog and saves file using FileSaveService
    public static void save(String txt) {
        initialize();
        try {
            // Show save dialog if no name is already given
            if (fc == null) {
                fc = fss.saveFileDialog(null, null,
                        new ByteArrayInputStream(txt.getBytes()), null);
                // file saved, done
                return;
            }
            // use this only when filename is known
            if (fc != null) {
                writeToFile(txt, fc);
            }
        } catch (IOException ioe) {
            ioe.printStackTrace(System.out);
        }
    }

    // displays saveAsFileDialog and saves file using FileSaveService
    public static void saveAs(String txt) {
        initialize();
        try {
            if (fc == null) {
                // If not already saved. Save-as is like save
                save(txt);
            } else {
                fc = fss.saveAsFileDialog(null, null, fc);
                save(txt);
            }
        } catch (IOException ioe) {
            ioe.printStackTrace(System.out);
        }
    }

    private static void writeToFile(String txt, FileContents fc) throws IOException {
        int sizeNeeded = txt.length() * 2;
        if (sizeNeeded > fc.getMaxLength()) {
            fc.setMaxLength(sizeNeeded);
        }
        BufferedWriter os = new BufferedWriter(new OutputStreamWriter(fc.getOutputStream(true)));
        os.write(txt);
        os.close();
    }

    private static String readFromFile(FileContents fc) throws IOException {
        if (fc == null) {
            return null;
        }
        BufferedReader br = new BufferedReader(new InputStreamReader(fc.getInputStream()));
        StringBuffer sb = new StringBuffer((int) fc.getLength());
        String line = br.readLine();
        while (line != null) {
            sb.append(line);
            sb.append("\n");
            line = br.readLine();
        }
        br.close();
        return sb.toString();
    }
}

注意: To compile Java code that has a reference to classes in the javax.jnlp package, include <your JDK path>/jre/lib/javaws.jar in your classpath. At runtime, the Java Runtime Environment software automatically makes these classes available to RIAs.

Download source codefor the Text Editor Applet example to experiment further.