Question

我有以下问题。我想创建一个无需绕过弯路即可直接访问SQL Server数据库的应用程序。

但是，我发现错误不是应用程序崩溃的原因。并不是真正可以帮助我摆脱困境的东西。我很感谢你的帮助每当我按下连接按钮时，我的应用就会在模拟器中崩溃。

MainActivity.java

package com.example.sql_query;

imports;

public class MainActivity extends Activity {

    Button btnConnect, btnAdd, btnDel, btnCom;
    TextView tv;
    EditText etINSERT;
    Connection conn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        tv = (TextView) findViewById(R.id.tvDs);
        etLieferkennung = (EditText) findViewById(R.id.etLieferkennung);

        btnAdd = (Button) findViewById(R.id.btnAdd);
        btnAdd.setEnabled(false);

        btnConnect = (Button) findViewById(R.id.btnConnect);
        btnConnect.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Query();
                btnConnect.setEnabled(false);
                btnAdd.setEnabled(true);
            }
        });

        btnAdd = (Button) findViewById(R.id.btnAdd);
        btnAdd.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                connect();
                PreparedStatement comm;
                try {
                    comm = conn.prepareStatement("insert into DATABASE("
                            + "COLOUMN) values(?)");
                    comm.setString(1, etINSERT.getText().toString());
                    comm.executeUpdate();
                } catch (SQLException e) {
                    tv.setText(e.toString());
                }
                Query();
            }
        });
    }



    public void connect() {
        try {
            StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder()
                    .permitAll().build();
            StrictMode.setThreadPolicy(policy);
            Class.forName("net.sourceforge.jtds.jdbc.Driver");
            conn = DriverManager.getConnection(""
                    + "jdbc:jtds:sqlserver://IP/DATABASENAME;"
                    + "user=EXAMPLE;password=EXAMPLE;");

        } catch (Exception e) {
            tv.setText(e.toString());
        }
    }

    public void Query() {
        connect();
        Statement comm;
        try {
            comm = conn.createStatement();
            ResultSet rs = comm.executeQuery("SELECT COLOUMNS FROM DATABASE Order by ID");
            String msg = "";
            while (rs.next()) {
                msg += "\nID: " + rs.getInt("ID") + " Lieferkennung: "
                        + rs.getString("Lieferkennung") + " Datum: "
                        + rs.getString("Datum");
            }
            tv.setText(msg);
        } catch (SQLException e) {
            tv.setText(e.toString());
        }
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
    <LinearLayout
        android:id="@+id/LinearLayout1"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:padding="8dp"
        android:background="@color/costum"
        android:orientation="vertical"
        tools:context="${relativePackage}.${activityClass}" >
        <EditText
            android:id="@+id/etINSERT"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginBottom="4dp"
            android:background="#eeeeee"
            android:hint="INSERT"
            android:padding="4dp"
            android:textColor="#000000"
            android:textSize="24dp">
            <requestFocus />
        </EditText>
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:padding="4dp">
            <Button
                android:id="@+id/btnConnect"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:backgroundTint="@color/costum"
                android:text="Connect" />
            <Button
                android:id="@+id/btnAdd"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:backgroundTint="@color/costum"
                android:text="Add new" />
            <Button
                android:id="@+id/btnDel"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:backgroundTint="@color/Tumeric"
                android:text="Delete Old" />
            <Button
                android:id="@+id/btnCom"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:backgroundTint="@color/Tumeric"
                android:text="Compare" />
        </LinearLayout>
        <TextView
            android:id="@+id/tvDs"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:background="#ffffff"
            android:padding="8dp"
            android:text=""
            android:textAppearance="?android:attr/textAppearanceMedium" />
    </LinearLayout>
</android.support.constraint.ConstraintLayout>

这是错误消息

EAndroidRuntime FATAL EXCEPTION main
Process com.example.sql_query, PID 19201
java.lang.NullPointerException Attempt to invoke interface method 'java.sql.Statement java.sql.Connection.createStatement()' on a null object reference
    at com.example.sql_query.MainActivity.Query(MainActivity.java86)
    at com.example.sql_query.MainActivity$1.onClick(MainActivity.java40)
    at android.view.View.performClick(View.java6256)
    at android.view.View$PerformClick.run(View.java24701)
    at android.os.Handler.handleCallback(Handler.java789)
    at android.os.Handler.dispatchMessage(Handler.java98)
    at android.os.Looper.loop(Looper.java164)
    at android.app.ActivityThread.main(ActivityThread.java6541)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java240)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java767)

希望你能帮助我。

Answer 1

如果在connect()函数中Query()调用失败，则conn未初始化。因此，调用conn.createStatement()会引发NullPointerException。

通常，仅通过记录/显示错误消息并继续来处理异常是一个非常糟糕的主意。您应该停止常规控制流，以使对象处于有效状态。例如，如果连接失败，则可以停止Query()方法。点击处理程序也是如此。

您可以通过在connect中引发异常并在调用该函数的地方对其进行处理来处理错误：

    public void connect() throws Exception {
        // ...
        conn = DriverManager.getConnection(/* ... */);
    }

    public void Query() {
        try {
            connect();
        } catch (Exception e) {
            // handle the error (i.e. show error message)
            return;
        }
        // ...
        try {
            comm = conn.createStatement();
            // ...
        } catch (SQLException e) {
            tv.setText(e.toString());
        }
    }

或者，您可以检查连接呼叫是否按预期工作：

    public void Query() {
        connect();
        if (conn == null) {
            // connection could not be initialized, the error message is 
            // set in connect()
            return;
        }
        Statement comm;
        try {
            comm = conn.createStatement();
            // ...
        } catch (SQLException e) {
            tv.setText(e.toString());
        }
    }

如何修复特定的Android应用程序崩溃？

1 个答案: