我在这方面花了很长时间，但还没有成功。在我的应用程序中捕获图像并发送到服务器，我在下面得到NullPointerException;
java.lang.NullPointerException: 0
at Files.CameraMIDlet.snap(CameraMIDlet.java:120)
at Files.CameraForm.commandAction(CameraForm.java:116)
at javax.microedition.lcdui.Display$ChameleonTunnel.callScreenListener(), bci=46
at com.sun.midp.chameleon.layers.SoftButtonLayer.processCommand(), bci=74
at com.sun.midp.chameleon.layers.SoftButtonLayer.soft2(), bci=173
at com.sun.midp.chameleon.layers.SoftButtonLayer.keyInput(), bci=78
at com.sun.midp.chameleon.CWindow.keyInput(), bci=38
at javax.microedition.lcdui.Display$DisplayEventConsumerImpl.handleKeyEvent(), bci=17
at com.sun.midp.lcdui.DisplayEventListener.process(), bci=277
at com.sun.midp.events.EventQueue.run(), bci=179
at java.lang.Thread.run(Thread.java:722)
错误发生在CameraMIDlet的byte[] image = videoControl.getSnapshot("encoding = jpeg");和CameraForm的midlet.snap();，在下面的代码中。
CameraForm的代码在这里：
package Files;
import javax.microedition.media.*;
import javax.microedition.lcdui.*;
import javax.microedition.media.control.*;
import java.io.IOException;
class CameraForm extends Form implements CommandListener {
private final CameraMIDlet midlet;
private final Command exitCommand;
private Command captureCommand = null;
private Command showImageCommand = null;
private Player player = null;
private static VideoControl videoControl = null;
private boolean active = false;
private StringItem messageItem;
public CameraForm(CameraMIDlet midlet) {
super("Camera");
this.midlet = midlet;
messageItem = new StringItem("Message", "start");
append(messageItem);
exitCommand = new Command("EXIT", Command.EXIT, 1);
addCommand(exitCommand);
setCommandListener(this);
try {
//creates a new player and set it to realize
player = Manager.createPlayer("capture://video");
player.realize();
//Grap the Video control and set it to the current display
videoControl = (VideoControl) (player.getControl("VideoControl"));
if (videoControl != null) {
append((Item) (videoControl.initDisplayMode(
VideoControl.USE_GUI_PRIMITIVE, null)));
captureCommand = new Command("CAPTURE", Command.SCREEN, 1);
addCommand(captureCommand);
messageItem.setText("OK");
} else {
messageItem.setText("No video control");
}
} catch (IOException ioe) {
messageItem.setText("IOException: " + ioe.getMessage());
} catch (MediaException me) {
messageItem.setText("Media Exception: " + me.getMessage());
} catch (SecurityException se) {
messageItem.setText("Security Exception: " + se.getMessage());
}
}
* the video should be visualized on the sreen
* therefore you have to start the player and set the videoControl visible
synchronized void start() {
if (!active) {
try {
if (player != null) {
player.start();
}
if (videoControl != null) {
videoControl.setVisible(true);
//midlet.snap();
}
} catch (MediaException me) {
messageItem.setText("Media Exception: " + me.getMessage());
} catch (SecurityException se) {
messageItem.setText("Security Exception: " + se.getMessage());
}
active = true;
}
}
* to stop the player. First the videoControl has to be set invisible
* than the player can be stopped
synchronized void stop() {
if (active) {
try {
if (videoControl != null) {
videoControl.setVisible(false);
}
if (player != null) {
player.stop();
}
} catch (MediaException me) {
messageItem.setText("Media Exception: " + me.getMessage());
}
active = false;
}
}
* on the captureCommand a picture is taken and transmited to the server
public void commandAction(Command c, Displayable d) {
if (c == exitCommand) {
midlet.cameraFormExit();
} else {
if (c == captureCommand) {
midlet.snap();
}
}
}
}
CameraMIDlet的代码如下：
package Files;
import java.io.*;
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import javax.microedition.io.*;
import javax.microedition.media.control.*;
import java.io.IOException;
import javax.microedition.media.MediaException;
public class CameraMIDlet extends MIDlet {
private CameraForm cameraSave = null;
private DisplayImage displayImage = null;
CameraForm captureThread;
private static VideoControl videoControl;
private StringItem messageItem;
public CameraMIDlet() {
}
/*
* startApp()
* starts the MIDlet and generates cameraSave, displayImage, database
*
**/
public void startApp() {
Displayable current = Display.getDisplay(this).getCurrent();
if (current == null) {
//first call
cameraSave = new CameraForm(this);
displayImage = new DisplayImage(this);
Display.getDisplay(this).setCurrent(cameraSave);
cameraSave.start();
} else {
//returning from pauseApp
if (current == cameraSave) {
cameraSave.start();
}
Display.getDisplay(this).setCurrent(current);
}
}
public void pauseApp() {
if (Display.getDisplay(this).getCurrent() == cameraSave) {
cameraSave.stop();
}
}
public void destroyApp(boolean unconditional) {
if (Display.getDisplay(this).getCurrent() == cameraSave) {
cameraSave.stop();
}
}
private void exitRequested() {
destroyApp(false);
notifyDestroyed();
}
void cameraFormExit() {
exitRequested();
}
/**
* restart the camera again
*
*/
void displayCanvasBack() {
Display.getDisplay(this).setCurrent(cameraSave);
cameraSave.start();
}
/**
* the byte[] of the image should be transmitted to a server
*
**/
void buildHTTPConnection(byte[] byteImage) {
displayImage.setImage(byteImage);
Display.getDisplay(this).setCurrent(displayImage);
HttpConnection hc = null;
OutputStream out = null;
try {
//enode the image data by the Base64 algorithm
String stringImage = Base64.encode(byteImage);
// URL of the Sevlet
String url = new String(
"http://ip-adress:8080/C:/Users/HASENDE/Documents/NetBeansProjects/Mobile/pics");
// Obtain an HTTPConnection
hc = (HttpConnection) Connector.open(url);
// Modifying the headers of the request
hc.setRequestMethod(HttpConnection.POST);
// Obtain the output stream for the HttpConnection
out = hc.openOutputStream();
out.write(stringImage.getBytes());
} catch (IOException ioe) {
StringItem stringItem = new StringItem(null, ioe.toString());
} finally {
try {
if (out != null)
out.close();
if (hc != null)
hc.close();
} catch (IOException ioe) {
}
}
// ** end network
}
/**
* stop the camera, show the captured image and transmit the image to a server
**/
void transmitImage(byte[] image) {
cameraSave.stop();
Display.getDisplay(this).setCurrent(displayImage);
buildHTTPConnection(image);
}
public void snap(){
try {
byte[] image = videoControl.getSnapshot("encoding = jpeg");
transmitImage(image);
messageItem.setText("Ok");
} catch (MediaException me) {
messageItem.setText("Media Exception: " + me.getMessage());
}
}
}
答案 0 :(得分：2)
通过识别抛出NPE的语句，您可以99％接近找到错误：
byte[] image = videoControl.getSnapshot("encoding = jpeg");
上述语句中的NPE表示videoControl为空。现在，如果您仔细观察它，您可能会注意到在CameraMIDlet中，videoControl初始化为null并且永远不会更改为其他任何东西 - 这就是您获得NPE的原因。顺便说一句，从CameraForm代码看起来你打算使用那里定义的videoControl对象，不是吗。
旁注。 CameraForm似乎被设计为在多个线程中使用（有synchronized修饰符） - 如果是这种情况，最好确保也以同步方式从中获取videoControl。同样在这种情况下，在volatile标志的定义中添加active修饰符：
private volatile boolean active = false; // in CameraForm
答案 1 :(得分：0)
用于捕获照片使用画布而不是Form， 检查以下Photo Capture的代码
public class ImageCaptureCanvas extends Canvas {
UrMidlet midlet;
VideoControl videoControl;
Player player;
SnapShotCanvas snap;
private Display display;
public ImageCaptureCanvas(UrMidlet midlet) throws MediaException {
this.midlet = midlet;
this.display = Display.getDisplay(midlet);
this.setFullScreenMode(true);
try {
player = Manager.createPlayer("capture://image");
player.realize();
videoControl = (VideoControl) player.getControl("VideoControl");
} catch (Exception e) {
dm(e.getClass().getName());
}
videoControl.initDisplayMode(VideoControl.USE_DIRECT_VIDEO, this);
try {
videoControl.setDisplayLocation(0,0);
videoControl.setDisplaySize(getWidth(), getHeight());
} catch (MediaException me) {
try {
videoControl.setDisplayFullScreen(true);
} catch (MediaException me2) {
}
}
dm("icc10");
videoControl.setVisible(true);
dm("icc11");
player.start();
this.display.setCurrent(this);
}
public void dm(String message) {
Form form = new Form("Error");
form.append(message);
display.setCurrent(form);
}
public void paint(Graphics g) {
}
protected void keyPressed(int keyCode) {
boolean prv=false;
int actn=getGameAction(keyCode);
switch (keyCode) {
case KEY_NUM5:
prv=true;
Thread t = new Thread() {
public void run() {
try {
byte[] raw = videoControl.getSnapshot(null);
Image image = Image.createImage(raw, 0, raw.length);
snap = new SnapShotCanvas(image);
display.setCurrent(snap);
} catch (Exception e) {
dm(e.getClass().getName() + " " + e.getMessage());
}
}
};
t.start();
break;
}
if(!prv){
switch (actn) {
case Canvas.FIRE:
Thread t1 = new Thread() {
public void run() {
try {
byte[] raw = videoControl.getSnapshot(null);
Image image = Image.createImage(raw, 0, raw.length);
snap = new SnapShotCanvas(image);
display.setCurrent(snap);
} catch (Exception e) {
dm(e.getClass().getName() + " " + e.getMessage());
}
}
};
t1.start();
break;
}
}
}
}
SnapShot Canvas Code
class SnapShotCanvas extends Canvas {
private Image image;
public SnapShotCanvas(Image image) {
this.image = image;
setFullScreenMode(true);
}
public void paint(Graphics g) {
g.drawImage(image, getWidth() / 2, getHeight() / 2, Graphics.HCENTER | Graphics.VCENTER);
}
}