Question

尝试将数据添加到ArrayList时，我一直收到空点异常。我已经在线查看并且所有答案都说确保你初始化了ArrayList，我做了，但我仍然得到一个空指针异常。有什么想法吗？

package resources;

import java.sql.*;
import java.util.*;
import java.io.*;
import javax.naming.*;
import javax.servlet.http.*;

public class CoursesSupportBean
{

    private String url;
    private String DataSourceName;
    String error;
    private List<Object> allData = new ArrayList<Object>();

    public String course_name;
    public int courseId;
    Object obj=new Object();

    public CoursesSupportBean() {
        url="";
        DataSourceName="";
        error="";
        allData = null;
        course_name="";
        courseId=0;
        obj=null;
    }


    public void setObj(Object obj) {
        this.obj = obj;
    }
    public String getError() {
        return error;
    }

    public void setUrl(String url){
        this.url = url;
    }

    public void setDataSourceName(String DataSourceName){
        this.DataSourceName=DataSourceName;
    }

    public List getDb_Data()
    {
        String framework = "embedded";
        String driver = "org.apache.derby.jdbc.EmbeddedDriver";
        String protocol = "jdbc:derby:";
        //HttpSession session = req.getSession();
        //String url = (String)session.getAttribute("url");
        //String DataSourceName = (String)session.getAttribute("DataSourceName");
        //res.setContentType("text/html"); 
        //PrintWriter out = res.getWriter();

        Context ctx = null;
        java.sql.Connection conn = null;
        Hashtable ht = new Hashtable();
        ht.put(Context.INITIAL_CONTEXT_FACTORY,"weblogic.jndi.WLInitialContextFactory");
        ht.put(Context.PROVIDER_URL,url);
        ArrayList statements = new ArrayList(); // list of Statements, PreparedStatements
        PreparedStatement psInsert = null;
        PreparedStatement psUpdate = null;
        Statement s = null;
        ResultSet rs = null;
        int i=0;
        try 
        {
            ctx = new InitialContext(ht);
            try {
                ctx.createSubcontext(DataSourceName);
            }
            catch (NameAlreadyBoundException e) {
            // Subcontext already exists.
            // Note that WebLogic's Context implementation does not throw this
            // exception if the name is already bound to an identical object.
            }
            String dbName = "JHU";
            try {
                javax.sql.DataSource ds = (javax.sql.DataSource) ctx.lookup(DataSourceName);
                conn = ds.getConnection();
                conn.setAutoCommit(false);
                s = conn.createStatement();
                statements.add(s);
            }
            catch (SQLException e) {
                System.out.println("----- SQLException -----");
                System.out.println("  SQL State:  " + e.getSQLState());
                System.out.println("  Error Code: " + e.getErrorCode());
                System.out.println("  Message:    " + e.getMessage());
            }
        }
        catch (NamingException e) {
            System.out.println("Naming Exception");
        }
        try 
        {   
            rs = s.executeQuery("SELECT * FROM COURSES");
            if (rs == null)
            {
                System.out.println("No rows in ResultSet");
            }
            while (rs.next())
            {
                DataFields d=new DataFields(rs.getInt(1), rs.getString(2));                    
                System.out.println("Course data: "+ rs.getInt(1) + " " + rs.getString(2));
                this.allData.add(i,d);
                i++;
            }
        } 
        catch (SQLException e) {
            System.out.println("----- SQLException -----");
            System.out.println("  SQL State:  " + e.getSQLState());
            System.out.println("  Error Code: " + e.getErrorCode());
            System.out.println("  Message:    " + e.getMessage());
        }
        try {
            if (rs != null) {
                rs.close();
                rs = null;
            }
        } catch (SQLException e) {
            System.out.println("----- SQLException -----");
            System.out.println("  SQL State:  " + e.getSQLState());
            System.out.println("  Error Code: " + e.getErrorCode());
            System.out.println("  Message:    " + e.getMessage());
        }

        // Statements and PreparedStatements
        int c=0;
        while (!statements.isEmpty()) {
            // PreparedStatement extend Statement
            Statement st = (Statement)statements.remove(c);
            try {
                if (st != null) {
                    st.close();
                    st = null;
                }
            } catch (SQLException e) {
                System.out.println("----- SQLException -----");
                System.out.println("  SQL State:  " + e.getSQLState());
                System.out.println("  Error Code: " + e.getErrorCode());
                System.out.println("  Message:    " + e.getMessage());
            }
        }

        //Connection
        try {
            if (conn != null) {
                conn.close();
                conn = null;
            }
        } catch (SQLException e) {
            System.out.println("----- SQLException -----");
            System.out.println("  SQL State:  " + e.getSQLState());
            System.out.println("  Error Code: " + e.getErrorCode());
            System.out.println("  Message:    " + e.getMessage());
        }
       return this.allData;
    }
    public int getCourseId()
    {
        this.courseId=((DataFields)obj).courseId;
        return this.courseId;
    }
    public String getCourse_Name() {
        this.course_name=((DataFields)obj).course_name;
        return this.course_name;
    }

    public class DataFields
    {
        public String course_name;
        public int courseId;

        public DataFields(int courseId, String  course_name)
        {
            this.courseId=courseId;
            this.course_name=course_name;
        }
    }

}

记录Cat错误

在resources.CoursesSupportBean.getDb_Data（CoursesSupportBean.java:109）的java.lang.NullPointerException

Answer 1

您正在构造函数中将allData ArrayList设置为null。删除这一行：

allData = null;

从那里开始。通常，如果要在构造函数中初始化变量，只需在构造函数外部的类中声明它们，并仅在构造函数中初始化它们。换句话说，将以上allData = null;行替换为：

allData = new ArrayList<Object>();

在构造函数之外替换此行：

private List<Object> allData = new ArrayList<Object>();

用这个：

private List<Object> allData;

Answer 2

ArrayList由定义初始化为字段，此处为：

private List<Object> allData = new ArrayList<Object>();

但是你似乎在构造函数中将allData设置为null：

public CoursesSupportBean() {
    url="";
    DataSourceName="";
    error="";
    allData = null;
    course_name="";
    courseId=0;
    obj=null;
}

所以你得到一个空指针异常，因为你正在设置已经初始化为null的ArrayList引用allData（不再指向创建的ArrayList）

只需删除此行：

allData = null;

通常，您初始化构造函数中的所有成员，但集合的初始化通常保留在字段定义中，除非它是一个昂贵的操作，因此尽可能地延迟。

ArrayList抛出空指针异常

2 个答案: