java - 线程 "Thread-5"java.lang.NullPointerException 中的异常

Question

在运行代码时我遇到了这个错误:

Exception in thread "Thread-5" java.lang.NullPointerException at 
com.example.example1.service.RecordData.run(RecordData.java:87)

这是我的程序:

public class SearchParser {

public static void main(String[] args) {
    String url = "jdbc:mysql://localhost:3306/";
    String dbName = "share";
    String driver = "com.mysql.jdbc.Driver";
    String userName = "root";
    String password = "";

    try {
        List<RecordData> list = new ArrayList<RecordData>();
        Connection conn = DriverManager.getConnection(url + dbName,
                userName, password);

        Statement stmt = conn.createStatement();
        ResultSet mysqldata = stmt
                .executeQuery("select * from product_links");
        while (mysqldata.next()) {

            RecordData data = new RecordData();
            data.setId(mysqldata.getInt("id"));
            data.setDomain_name(mysqldata.getString("domain_name"));
            data.setCommon_link_name(mysqldata
                    .getString("common_link_name"));
            data.setHead_element_pattern(mysqldata
                    .getString("head_element_pattern"));
            data.setProduct_title_pattern(mysqldata
                    .getString("product_title_pattern"));
            data.setProduct_link_pattern(mysqldata
                    .getString("product_link_pattern"));
            data.setCurrent_price_pattern(mysqldata
                    .getString("current_price_pattern"));
            data.setImage_src_pattern(mysqldata
                    .getString("image_src_pattern"));
            data.setDecscription_pattern(mysqldata
                    .getString("decscription_pattern"));
            list.add(data);
        }
        for (RecordData data : list) {
            RecordData p = new RecordData();
            p.start();
        }

    } catch (Exception e) {
        System.out.print(e);
    }

}

它基本上是一个从数据库中获取数据的程序。

public class RecordData extends Thread {
    int id;
    String domain_name, common_link_name, head_element_pattern, product_title_pattern, product_link_pattern, current_price_pattern, image_src_pattern,
            decscription_pattern;
    public RecordData data;

    public RecordData() {
        this.data = data;
    }

    public long getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getDomain_name() {
        return domain_name;
    }

    public void setDomain_name(String domain_name) {
        this.domain_name = domain_name;
    }

    public String getCommon_link_name() {
        return common_link_name;
    }

    public void setCommon_link_name(String common_link_name) {
        this.common_link_name = common_link_name;
    }

    public String getHead_element_pattern() {
        return head_element_pattern;
    }

    public void setHead_element_pattern(String head_element_pattern) {
        this.head_element_pattern = head_element_pattern;
    }

    public String getProduct_title_pattern() {
        return product_title_pattern;
    }

    public void setProduct_title_pattern(String product_title_pattern) {
        this.product_title_pattern = product_title_pattern;
    }

    public String getProduct_link_pattern() {
        return product_link_pattern;
    }

    public void setProduct_link_pattern(String product_link_pattern) {
        this.product_link_pattern = product_link_pattern;
    }

    public String getCurrent_price_pattern() {
        return current_price_pattern;
    }

    public void setCurrent_price_pattern(String current_price_pattern) {
        this.current_price_pattern = current_price_pattern;
    }

    public String getImage_src_pattern() {
        return image_src_pattern;
    }

    public void setImage_src_pattern(String image_src_pattern) {
        this.image_src_pattern = image_src_pattern;
    }

    public String getDecscription_pattern() {
        return decscription_pattern;
    }

    public void setDecscription_pattern(String decscription_pattern) {
        this.decscription_pattern = decscription_pattern;
    }

    public void run() {
        System.out.println(data.domain_name);
    }
}

Answer 1

因为这个

public RecordData() {
    this.data = data;
}

什么都不做。您正在为自己分配一个字段。因为它以 null 开始，所以它也以 null 结束。

我不知道你想要达到什么目的，但你可能想为 data 分配一些其他值。

public RecordData(RecordData data) {
    this.data = data;
}