The Java Tutorials have been written for JDK 8.Java教程是为JDK 8编写的。Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.本页中描述的示例和实践没有利用后续版本中引入的改进，并且可能使用不再可用的技术。See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.有关Java SE 9及其后续版本中更新的语言特性的摘要，请参阅Java语言更改。
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.有关所有JDK版本的新功能、增强功能以及已删除或不推荐的选项的信息，请参阅JDK发行说明。
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.
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();
}
}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 code for the Text Editor Applet example to experiment further.