如果我使用Debug（Eclipse）运行以下代码（JAVAfx），我会得到一个Exception。 正常运行正常。 我在运行服务器时运行它，服务器识别此客户端。
这是一个例外：
Exception in Application start method
djava.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(Unknown Source)
at com.sun.javafx.application.LauncherImpl.launchApplication(Unknown Source)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at sun.launcher.LauncherHelper$FXHelper.main(Unknown Source)
Caused by: java.lang.RuntimeException: Exception in Application start method
at com.sun.javafx.application.LauncherImpl.launchApplication1(Unknown Source)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$150(Unknown Source)
at com.sun.javafx.application.LauncherImpl$$Lambda$50/24970616.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
at javaNetFinal.Client.delColumnPane(Client.java:122)
at javaNetFinal.Client.updateStage(Client.java:333)
at javaNetFinal.Client.start(Client.java:75)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$156(Unknown Source)
at com.sun.javafx.application.LauncherImpl$$Lambda$85/17408914.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$169(Unknown Source)
at com.sun.javafx.application.PlatformImpl$$Lambda$47/16153462.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl.lambda$null$167(Unknown Source)
at com.sun.javafx.application.PlatformImpl$$Lambda$49/32223832.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$168(Unknown Source)
at com.sun.javafx.application.PlatformImpl$$Lambda$48/20929065.run(Unknown Source)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(Unknown Source)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$144(Unknown Source)
at com.sun.glass.ui.win.WinApplication$$Lambda$37/29854731.run(Unknown Source)
... 1 more
Exception running application javaNetFinal.Client
代码：
public class Client extends Application {
DynamicTable tbl = new DynamicTable(0,null,null);
PaneQuery paneQuery;
private Socket socket;
// Input and output streams from/to server
private ObjectInputStream fromServer;
private DataOutputStream toServer;
private String host = "localhost";
private BorderPane viewPane = new BorderPane(); // include: upperPaneOnViewPane (Queries, Filter) Table, edit
private BorderPane topViewPane = new BorderPane();
private BorderPane upperPaneOnViewPane = new BorderPane(); // include: Queries, Filter
private int columnCount;
private Student[] rows;
private String[] columnName;
public void start(Stage primaryStage) {
connectToServer();
refreshDefultTable();
BorderPane mainPane = new BorderPane();
// Top View
topViewPane.setPadding(new Insets(5,5,30,5));
topViewPane.setLeft(initRefreshTable()); // position of the first part in the top pane
//topViewPane.setStyle("-fx-border-color: black;");
// Center View
upperPaneOnViewPane.setPadding(new Insets(5,5,10,5));
//upperPaneOnViewPane.setLeft(paneQuery.createPane()); // Queries
upperPaneOnViewPane.setRight(setPaneFilterOnTable()); // Filter
viewPane.setTop(upperPaneOnViewPane); // Queries + Filter
viewPane.setBottom(setPaneEditTable()); // Edit Table
mainPane.setPadding(new Insets(5,5,5,5));
mainPane.setTop(topViewPane);
mainPane.setCenter(viewPane);
updateStage();
Scene scene = new Scene(mainPane, 700, 250);
primaryStage.setMaximized(true);
primaryStage.setTitle("----"); // Set the window title
primaryStage.setScene(scene); // Place the scene in the window
primaryStage.show(); // Display the window
primaryStage.setAlwaysOnTop(true);
}
public Pane initRefreshTable() {
GridPane initRefreshPane = new GridPane();
initRefreshPane.setVgap(4);
initRefreshPane.setHgap(10);
initRefreshPane.setPadding(new Insets(5,5,5,5));
Button btnInitial= new Button("Init DataBase");
btnInitial.setOnAction(e-> {
new ImportMySQLDB();
refreshDefultTable();
updateStage();
});
initRefreshPane.add(btnInitial,0,0);
Button btnRefresh= new Button("Refresh Table View");
btnRefresh.setOnAction(e-> {
System.out.println("dd");
refreshDefultTable();
// String sql = "SELECT * FROM STUDENT";
// sendAndGetFromServer(sql);
});
initRefreshPane.add(btnRefresh,1,0);
Label lblLastSave = new Label("Last Saved: ");
initRefreshPane.add(lblLastSave,0,1);
Label lblDateSave = new Label(new Date().toString());
initRefreshPane.add(lblDateSave,1,1);
return initRefreshPane;
}
public Pane delColumnPane() {
GridPane delColumnPane = new GridPane();
delColumnPane.setVgap(4);
delColumnPane.setHgap(10);
delColumnPane.setPadding(new Insets(5,5,5,5));
ObservableList<String> columnList =
FXCollections.observableArrayList(this.columnName.length>0? Student.getHeadField(this.columnName):null);
columnList.remove("ID");
ComboBox<String> cbColumnToDel = new ComboBox<String>(columnList);
Button btnDelColumn= new Button("Delete");
delColumnPane.add(cbColumnToDel, 0, 0);
delColumnPane.add(btnDelColumn,1,0);
cbColumnToDel.setPromptText("delete column...");
cbColumnToDel.setOnAction(e ->{
btnDelColumn.setDisable(false);
});
btnDelColumn.setDisable(true);
btnDelColumn.setOnAction(e ->{
// showDialog("\n\n Are you sure you want delete column: "+cbColumnToDel.getValue()+"?");
System.out.println(tbl.toString());
String sql = "ALTER TABLE STUDENT";
sql=sql +" DROP COLUMN "+Student.getField(cbColumnToDel.getValue());
sendAndGetFromServer(sql);
System.out.println(tbl.toString());
columnList.remove(Student.getField(cbColumnToDel.getValue()));
cbColumnToDel.setPromptText("delete column...");
btnDelColumn.setDisable(true);
updateStage();
System.out.println(tbl.toString());
});
return delColumnPane;
}
public Pane setPaneFilterOnTable() {
GridPane filterTable = new GridPane();
filterTable.setVgap(4);
filterTable.setHgap(10);
filterTable.setPadding(new Insets(5,5,5,5));
Label lblSort = new Label("Sort Columns By:");
filterTable.add(lblSort, 0, 0);
ObservableList<String> columnOptions =
FXCollections.observableArrayList(
//tbl.getColumnName()
);
ComboBox<String> cbFirstChoiseColumn = new ComboBox<String>(columnOptions);
cbFirstChoiseColumn.setPromptText("Select first column...");
cbFirstChoiseColumn.setPrefWidth(210);
filterTable.add(cbFirstChoiseColumn, 1, 0);
ComboBox<String> cbSecondChoiseColumn = new ComboBox<String>(columnOptions);
cbSecondChoiseColumn.setPromptText("Select second column...");
cbSecondChoiseColumn.setPrefWidth(210);
cbSecondChoiseColumn.setDisable(true);
cbFirstChoiseColumn.setOnAction(new EventHandler<ActionEvent>() {
@Override
public void handle(ActionEvent event) {
cbSecondChoiseColumn.setDisable(false);
}
});
filterTable.add(cbSecondChoiseColumn, 1, 1);
Label lblOrder = new Label("Order By:");
filterTable.add(lblOrder, 2, 0);
ObservableList<String> orderOptions =
FXCollections.observableArrayList(
"▲ Ascending", "▼ Descending" );
ComboBox<String> cbOrder = new ComboBox<String>(orderOptions);
cbOrder.setPromptText("Ascending");
cbOrder.setPrefWidth(150);
filterTable.add(cbOrder, 3, 0);
return filterTable;
}
public Pane setPaneEditTable() {
GridPane editTable = new GridPane();
editTable.setVgap(4);
editTable.setHgap(10);
editTable.setPadding(new Insets(15,5,15,5));
RadioButton rbEditRow = new RadioButton();
rbEditRow.setText("Edit");
editTable.add(rbEditRow,0,0);
rbEditRow.setSelected(true);
RadioButton rbAddRow = new RadioButton();
rbAddRow.setText("Add");
editTable.add(rbAddRow,1,0);
RadioButton rbDeleteRow = new RadioButton();
rbDeleteRow.setText("Delete");
editTable.add(rbDeleteRow,2,0);
rbEditRow.setOnAction(new EventHandler<ActionEvent>() {
@Override
public void handle(ActionEvent event) {
rbAddRow.setSelected(false);
rbDeleteRow.setSelected(false);
}
});
rbAddRow.setOnAction(new EventHandler<ActionEvent>() {
@Override
public void handle(ActionEvent event) {
rbEditRow.setSelected(false);
rbDeleteRow.setSelected(false);
}
});
rbDeleteRow.setOnAction(new EventHandler<ActionEvent>() {
@Override
public void handle(ActionEvent event) {
rbEditRow.setSelected(false);
rbAddRow.setSelected(false);
}
});
Button btnApply= new Button("Apply");
editTable.add(btnApply,0,1);
return editTable;
}
private void connectToServer(){
try{
// Create a socket to connect to the server
socket = new Socket(host, 8000);
// Create an output stream to send data to the server
toServer = new DataOutputStream(socket.getOutputStream());
// Create an input stream to receive data from the server
fromServer = new ObjectInputStream(socket.getInputStream());
}
catch (Exception ex){
ex.printStackTrace();
}
}
private void sendAndGetFromServer(String sqlQuery){
new Thread(() ->{
try{
System.out.println("a1");
// Send sql query to server
toServer.writeUTF(sqlQuery);
// Get notification from the server
this.rows = (Student[])fromServer.readObject();
this.columnCount= fromServer.readInt();
this.columnName= (String[])fromServer.readObject();
setRowsInTable();
}
catch(SocketException ex){
try{
socket.close();
}
catch (IOException e){
ex.printStackTrace();
}
}
catch (Exception ex){
ex.printStackTrace();
}
}).start();
System.out.println("end");
}
public void setRowsInTable(){
tbl = new DynamicTable(columnCount, rows, columnName);
tbl.buildTable();
Platform.runLater(new Runnable() {
@Override
public void run() {
viewPane.setCenter(tbl);
}
});
}
public void refreshDefultTable(){
String sql = "SELECT * FROM STUDENT";
sendAndGetFromServer(sql);
}
public void updateStage(){
topViewPane.setRight(delColumnPane()); // position of the second part in the top pane
viewPane.setCenter(tbl); // Table
}
public static void main(String[] args) {
launch(args);
}
}
有什么问题？ 我需要调试！
答案 0 :(得分：1)
问题是，为了在“变量”视图中实际显示某些内容（或者IDE中调用的任何内容 - 这里是Eclipse），调试器通常在对象上使用toString（）方法。
在类似你的情况下，调试导致NPE，原因通常是，你的一个对象是以非原子方式创建的，并且它不准备在你定义的断点处使用。这可以是例如在使用对象之前需要设置的一些字段。但是，由于您的调试器已经开始“使用”该对象，因此您遇到了麻烦。
这种错误只能通过适当强健的toString（）实现来解决，该实现考虑到对象（至少最初）处于不一致状态的事实。