Java 语句上的空指针异常 = conn.createStatement();
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/18756159/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
nullpointer exception on statement = conn.createStatement();
提问by C Sharper
I am using jdbc in android (Aware not recomended)
我在 android 中使用 jdbc（注意不推荐）
I have following piece of code:
我有以下一段代码：
public int CheckUser(String uName,String password)
{
try {
String sql="select uid from UserMaster where username='"+uName+"' and password='"+password+"'";
statement = conn.createStatement();
resultSet = statement.executeQuery(sql);
int value=0;
if (resultSet.next()) {
value=resultSet.getInt(1);
}
return value;
}
catch (Exception e)
{
e.printStackTrace();
return 0;
}
}
I gets null pointer exception on statement = conn.createStatement();
我得到空指针异常 statement = conn.createStatement();
Logcat:
日志猫：
09-12 11:04:14.543: W/KeyCharacterMap(483): No keyboard for id 0
09-12 11:04:14.553: W/KeyCharacterMap(483): Using default keymap: /system/usr/keychars/qwerty.kcm.bin
09-12 11:04:51.173: W/System.err(483): java.lang.NullPointerException
09-12 11:04:51.194: W/System.err(483): at com.example.messagesql.gaSQLConnect.CheckUser(gaSQLConnect.java:69)
09-12 11:04:51.213: W/System.err(483): at com.example.messagesql.Login$UserLogin.doInBackground(Login.java:109)
09-12 11:04:51.213: W/System.err(483): at com.example.messagesql.Login$UserLogin.doInBackground(Login.java:1)
09-12 11:04:51.225: W/System.err(483): at android.os.AsyncTask.call(AsyncTask.java:185)
09-12 11:04:51.233: W/System.err(483): at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:306)
09-12 11:04:51.233: W/System.err(483): at java.util.concurrent.FutureTask.run(FutureTask.java:138)
09-12 11:04:51.253: W/System.err(483): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1088)
09-12 11:04:51.253: W/System.err(483): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:581)
09-12 11:04:51.264: W/System.err(483): at java.lang.Thread.run(Thread.java:1019)
Plz help.
请帮忙。
I tried declaring Statement object locally but no effect, still error persists.
我尝试在本地声明 Statement 对象但没有效果，错误仍然存在。
Code of my file which calls this function:
调用此函数的文件代码：
public class Login extends Activity {
public static String resp = "";
public static String name = "";
String username;
String password;
EditText etLoginID;
EditText etPassword;
final DatabaseHandler db = new DatabaseHandler(this);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_login);
List<Contact> contacts = db.getAllContacts();
if(contacts.size() != 0){
for (Contact cn : contacts){
resp = cn.getID();
name = cn.getName();
}
// Intent i=new Intent(getApplicationContext(),Message.class);
//startActivity(i);
}
etLoginID=(EditText)findViewById(R.id.etName);
etPassword=(EditText)findViewById(R.id.etPassword);
Button btnLogin =(Button)findViewById(R.id.btnLogin);
final gaSQLConnect con=new gaSQLConnect();
btnLogin.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View arg0) {
// TODO Auto-generated method stub
con.setConnection("AndroidDB", "sa", "ok");
username=etLoginID.getText().toString();
password=etPassword.getText().toString();
String[] values=new String[2];
values[0]=username;
values[1]=password;
UserLogin ul=new UserLogin();
ul.execute(values);
//Intent i= new Intent(getApplicationContext(),Messages.class);
//startActivity(i);
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.login, menu);
return true;
}
private class UserLogin extends AsyncTask<String[], Void, String>{
final gaSQLConnect con=new gaSQLConnect();
String[] values;
@Override
protected String doInBackground(String[]... params) {
// TODO Auto-generated method stub
values=params[0];
String result=null;
int uid;
db.deleteContact(new Contact(resp));
if (!values[0].isEmpty() && !values[1].isEmpty())
{
uid=con.CheckUser(etLoginID.getText().toString(), etPassword.getText().toString());
return Integer.toString(uid) ;
}
else{
uid=0;;
}
return Integer.toString(uid) ;
}
protected void onPostExecute(String result){
Toast.makeText(getApplicationContext(),result, Toast.LENGTH_LONG).show();
db.addContact(new Contact(result, values[0]));
Intent i=new Intent(getApplicationContext(),Messages.class);
startActivity(i);
}
}
Above is the code of file from which i calls the function.
上面是我调用函数的文件代码。
In this onClick method of login i have setted connection.
在这个 onClick 登录方法中，我已经设置了连接。
Then also why it does not persists through whole application?
那么为什么它不会在整个应用程序中持续存在？
Whole code of DBConnection class:
DBConnection 类的完整代码：
public class gaSQLConnect
{
String url ="";
Connection conn=null;
Statement statement=null;
ResultSet resultSet=null;
public ResultSet getMessages(int uid)
{
try
{
String sql="select mid,uid,message,rstamp from MessagesMaster where uid='"+uid+"' aand rstamp = 0 order by tstamp desc";
statement = conn.createStatement();
resultSet = statement.executeQuery(sql);
return resultSet;
}
catch(Exception ex)
{
}
return resultSet;
}
public void setConnection(String DBName,String UserName,String Password)
{
try {
Class.forName("net.sourceforge.jtds.jdbc.Driver").newInstance();
url ="jdbc:jtds:sqlserver://10.0.2.2:1433;instanceName=14GRAFICALI\MSSQLSERVER2008;DatabaseName="+DBName+";integratedSecurity=true;user="+UserName+";password="+Password+"";
conn =DriverManager.getConnection(url);
} catch (Exception e) {
e.printStackTrace();
}
}
int recFound=0;
public int CheckUser(String uName,String password)
{
try {
Statement statement=null;
ResultSet resultSet=null;
String sql="select uid from UserMaster where username='"+uName+"' and password='"+password+"'";
statement = conn.createStatement();
resultSet = statement.executeQuery(sql);
int value=0;
if (resultSet.next()) {
value=resultSet.getInt(1);
}
return value;
/* if (value > 0) {
recFound = 1;
} else {
recFound = 0;
}
if (recFound > 0)
{
return true;
} else {
return false;
}*/
}
catch (Exception e)
{
e.printStackTrace();
return 0;
}
}
public void UpdateMessage(String[] mlst) {
// TODO Auto-generated method stub
try
{
String sql="update messagesmaster set rstamp = 1 where mid in (" + mlst + ") ";
statement = conn.createStatement();
statement.executeUpdate(sql);
}
catch(Exception ex)
{
}
}
}
采纳答案by Juned Ahsan
As the exception trace says your connobject is null and hence calling any method on it will cause NullPointerException. You need to check your code where you are initializing your connobject to initialize it properly. Also it is always a good practice to put a null check to avoid null pointer exception.
由于异常跟踪表明您的conn对象为空，因此调用它的任何方法都会导致NullPointerException. 您需要检查初始化conn对象的代码以正确初始化它。此外，进行空检查以避免空指针异常始终是一个好习惯。
回答by newuser
your database connection is null. But you are trying to create a statement. So only you got the nullPointerException
您的数据库连接为空。但是您正在尝试创建一个声明。所以只有你得到了nullPointerException
if(conn != null)
and then proceed
然后继续
回答by Piyush
Whenever you are trying to create a statement that the Connectionmust not be null and if it is null then you have to check this
每当您尝试创建连接不能为空的语句时，如果它为空，则必须检查此
if(conn!=null){
//Create statement
{
回答by Dawood ibn Kareem
Your problem is that you're calling setConnectionon ONE gaSQLConnectobject, then calling CheckUseron a DIFFERENT gaSQLConnectobject, which has a different copy of the connvariable. Stop creating so many gaSQLConnectobjects.
您的问题是您正在调用setConnection一个gaSQLConnect对象，然后调用具有不同变量副本的CheckUserDIFFERENTgaSQLConnect对象conn。停止创建这么多gaSQLConnect对象。
回答by Raman B
use connection creation only once. If you are trying to pass any int value for success full connection rather Connection object , will get this error.
仅使用一次连接创建。如果您尝试传递任何 int 值以获得成功完全连接而不是 Connection object ，则会收到此错误。
