Java:将ResultSet查询为JSON [英] Java: Query ResultSet to JSON

问题描述

通过以下代码发送到Oracle DB的查询，并且应该以JSON的形式返回查询结果:

Query to Oracle DB being sent via following code and supposed to return the query result as JSON:

    Connection conn ;
    try {

        Class.forName("oracle.jdbc.driver.OracleDriver"); 
        String url = "jdbc:oracle:thin:@localhost:1521:dbname";     
        conn = DriverManager.getConnection(url,"username","pwd");  

        Statement stmt = conn.createStatement();
        ResultSet rs = stmt.executeQuery("SELECT * FROM table4 where ID = '5'");


        while (rs.next()) {
            String s = rs.getString("*");
            response.setContentType("application/json");
            response.setCharacterEncoding("UTF-8");
            response.getWriter().write(s);

        }

        conn.close();


    } catch (Exception e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } 

但是，String s的值始终为null.

However, the value of String s is always null.

我去过解决方案此处，但是它没有从表中选择*无效.

I've been to the solution here but it doesn't work for selecting * from the table.

推荐答案

要输出JSON，您需要首先将数据累积到List<Map<String, Object>>中.

To output JSON, you want to accumulate your data into a List<Map<String, Object>> first.

使用ResultSetMetaData获取列数和列名.

List<Map<String, Object>> rows = new ArrayList<>();
ResultSetMetaData rsmd = rs.getMetaData();
int columnCount = rsmd.getColumnCount();

while (rs.next()) {
      // Represent a row in DB. Key: Column name, Value: Column value
      Map<String, Object> row = new HashMap<>();
      for (int i = 1; i <= columnCount; i++) {
           // Note that the index is 1-based
           String colName = rsmd.getColumnName(i);
           Object colVal = rs.getObject(i);
           row.put(colName, colVal);
      }
      rows.add(row);
}

// Write the list of rows to output
// Recommend to use jackson-ObjectMapper to streaming json directly to outputstream:
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.writeValue(response.getOutputStream(), rows);

要使用Jackson ObjectMapper，请向您的项目添加依赖项:

To use Jackson ObjectMapper, add dependency to your project:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.9.4</version>
</dependency>

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