我的Android应用程序连接到我的服务器，当我运行我的服务器程序时，它工作正常，但当它停止并且我的应用程序尝试连接到服务器时，我想向用户显示Toast消息或其他内容。但首先，当服务器脱机时，我必须阻止我的应用程序强行关闭。
这是我得到的错误
Process: project.sharethefare, PID: 27120
java.lang.RuntimeException: Unable to start activity ComponentInfo{project.sharethefare/project.sharethefare.Share}: java.lang.NullPointerException: Attempt to invoke virtual method 'void java.io.ObjectInputStream.close()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2658)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2725)
at android.app.ActivityThread.access$900(ActivityThread.java:172)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1422)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:5834)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1388)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1183)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void java.io.ObjectInputStream.close()' on a null object reference
at project.sharethefare.ServerConnect.run(ServerConnect.java:95)
at project.sharethefare.Share.onCreate(Share.java:51)
at android.app.Activity.performCreate(Activity.java:6221)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2611)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2725)
at android.app.ActivityThread.access$900(ActivityThread.java:172)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1422)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:5834)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1388)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1183)
这是与服务器建立连接的代码。它来自另一个班级
public class ServerConnect{
Socket requestSocket;
ObjectOutputStream out;
ObjectInputStream in;
double message;
ServerConnect(){}
String TAG = "ShareTheFare";
Share s;
HomeScreen home = new HomeScreen();
public static Double matchLocLat,matchLocLong,matchDesLat,matchDesLong;
void run(){
s = new Share();
double curlat = home.curLat; //Shanowen
double curlong =home.curLong;
double deslat = home.desLat; //Parnell Street
double deslong =home.desLong;
/*double curlat = -37.862200;
double curlong = 144.896377;
double deslat = -37.860845;
double deslong = 144.894252;*/
try{
//1. creating a socket to connect to the server
requestSocket = new Socket("178.62.125.141",4444 ); //178.62.125.141
System.out.println("Connected to localhost in port 2004");
//2. get Input and Output streams
out = new ObjectOutputStream(requestSocket.getOutputStream());
out.flush();
in = new ObjectInputStream(requestSocket.getInputStream());
int count=0;
//3: Communicating with the server
while(count<5){ //this should break from server when the user recieves the 4 long and lats from server
try{
message = (Double)in.readObject();
System.out.println("serverSent>" + message);
if(count==0){
//System.out.println("server>" + message);
sendMessage(curlat);
Log.d(TAG,"1");
sendMessage(curlong);
Log.d(TAG,"2");
sendMessage(deslat);
Log.d(TAG,"3");
sendMessage(deslong);
Log.d(TAG,"4");
}
if(message==1234.5){
//call the method to print out a toast saying no match found
}
else {
if (count == 1) {
matchLocLat = message;
}
if (count == 2) {
matchLocLong = message;
}
if (count == 3) {
matchDesLat = message;
}
if (count == 4) {
matchDesLong = message;
}
}
count++;
//System.out.println("Count: " + count);
}
catch(ClassNotFoundException classNot){
//S
}
}
}
catch(UnknownHostException unknownHost){
System.err.println("You are trying to connect to an unknown host!");
}
catch(IOException ioException){
ioException.printStackTrace();
}
finally{
//4: Closing connection
try{
in.close();
out.close();
requestSocket.close();
}
catch(IOException ioException){
ioException.printStackTrace();
}
}
}
void sendMessage(Double msg)
{
try{
out.writeObject(msg);
out.flush();
//System.out.println("client>" + msg);
}
catch(IOException ioException){
ioException.printStackTrace();
}
}
double getlat(){
return matchLocLat;
}
double getlong() {
return matchLocLong;
}
}
答案 0 :(得分：1)
在try块中添加空检查。你应该几乎总是检查无效 - 这只是一个好习惯。
例如：
try {
if (requestSocket != null) { // this will be null if your server is offline when you tried to connect
in.close();
out.close();
requestSocket.close();
}
} // other stuff
请注意，这并不能解决您的代码问题 - 这里还有另一个问题。
您可能会在之前在finally块中获得空指针异常，但由于您有一个finally块，因此您只能看到该块中的异常。
您希望在致电getInputStream()或getOutputStream()之前进行空检查 - 或者您可以抓住NPE并处理它。
答案 1 :(得分：-1)
尝试关闭时，输出流为空。将其初始化移到第一个try块之外应修复NullPointerException。