使用 java 注释生成 Postgresql table

Question

我正在研究注释和休眠的工作原理，以便更好地理解它。

我知道我的代码可能不是最好的质量，当我可以解决我的异常时我会修复它，我一直收到 NullPointerException，但我不明白为什么，我做了什么错了？

这个class中的主要只是为了找到问题的原因，我有另一个class将使用这个，从我的另一个class我可以将数据添加到我的 table，但我需要能够使用注释生成 table，这就是我获取 NullPointerException 的地方我无法生成我的 table

这是我的StackTrace

Exception in thread "main" java.lang.RuntimeException: java.lang.NullPointerException
at psybergate.grad2018.javafnds.javabasics.annotations.hw3.vendor.DatabaseManager.generateDatabase(DatabaseManager.java:84)
at psybergate.grad2018.javafnds.javabasics.annotations.hw3.vendor.DatabaseManager.main(DatabaseManager.java:40)
Caused by: java.lang.NullPointerException
at psybergate.grad2018.javafnds.javabasics.annotations.hw3.vendor.DatabaseManager.generateDatabase(DatabaseManager.java:82)
... 1 more

这是我的代码

package psybergate.grad2018.javafnds.javabasics.annotations.hw3.vendor;

import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;

import psybergate.grad2018.javafnds.javabasics.annotations.hw3.annotations.DomainClass;
import psybergate.grad2018.javafnds.javabasics.annotations.hw3.annotations.DomainProperty;
import psybergate.grad2018.javafnds.javabasics.annotations.hw3.developer.Customer;

/**
* @since 26 Apr 2018
* @author christiaan.dotze
*/
public class DatabaseManager {

private String addField = "";

private Connection connect = null;

private Statement statement = null;

private String primaryKey = "";

private String tableName = "";

public static void main(String[] args) {
    DatabaseManager manager = new DatabaseManager();
    try {
        Customer cust = new Customer();
        cust.setCustomerNum("11");
        cust.setFirstName("John");
        cust.setLastName("Doe");
        cust.setAge(27);
        cust.setDateOfBirth(1991);

        manager.generateDatabase(cust);
    } catch (ClassNotFoundException | NoSuchFieldException | SecurityException | SQLException ex) {
        throw new RuntimeException("An error occured", ex);
    }
}

public String getTableName(Object obj) {
    Annotation[] domainClassAnnotation = obj.getClass().getDeclaredAnnotations();
    for (Annotation annotation : domainClassAnnotation) {
        if (annotation instanceof DomainClass) {
            tableName = ((DomainClass) annotation).name();
        }
    }
    return tableName;
}

private String getDatabaseFields(Object obj) throws NoSuchFieldException, SecurityException {
    Field[] decladedFields = obj.getClass().getDeclaredFields();
    for (Field field : decladedFields) {
        Annotation[] annotations = field.getDeclaredAnnotations();
        for (Annotation annotation : annotations) {
            if (annotation instanceof DomainProperty) {
                DomainProperty myAnnotation = (DomainProperty) annotation;
                if (myAnnotation.primaryKey() == true) primaryKey = myAnnotation.name();
                if (myAnnotation.unique() == false) {
                    addField += myAnnotation.name() + " " + myAnnotation.dataType() + ",\n";
                } else {
                    addField += myAnnotation.name() + " " + myAnnotation.dataType() + " " + "NOT NULL" + ",\n";
                }
            }
        }
    }
    addField += "PRIMARY KEY(" + primaryKey + ")\n";
    return addField;
}

public void generateDatabase(Object obj)
        throws SQLException, ClassNotFoundException, NoSuchFieldException, SecurityException {
    createConnection();
    getTableName(obj);
    getDatabaseFields(obj);
    try {
        statement.executeUpdate("CREATE TABLE " + getTableName(obj) + "(\n" + getDatabaseFields(obj) + ")");
    } catch (Exception ex) {
        throw new RuntimeException(ex);
    }
}

private void createConnection() throws ClassNotFoundException, SQLException {
    Class.forName("org.postgresql.Driver");
    connect = DriverManager.getConnection("jdbc:postgresql://localhost:5432/", "postgres", "1234");
}

public int add(Object obj, String data)
        throws NoSuchFieldException, SecurityException, SQLException, ClassNotFoundException {
    createConnection();
    statement = connect.createStatement();
    return statement.executeUpdate("INSERT INTO " + getTableName(obj) + " VALUES (" + data + ")");
}
}

Answer 1

您在 'generateDatabase' 方法中有空对象 'statement'。当 'add' 从不运行时，'statement' 字段在 'add' 方法中初始化。

尽量放

statement = connect.createStatement();

在

之后

createConnection();

在 'generateDatabase' 方法中。

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