Jackson.DataBind.JsonMappingException：已调用getWriter（）

Question

使用jackson回写响应时发生异常。在网上找不到任何解决方案。我不确定错误的原因是什么。尝试刷新响应，但仍然是相同的错误

import com.In10s.DBProperties.OracleDBConnection;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.IOException;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.json.JSONArray;
import org.json.JSONObject;

public class GetTable extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        JSONObject jsonResponse = new JSONObject();

        try {
            JSONArray data = new JSONArray();
            Connection con = OracleDBConnection.getConnection();
            String query = "Select * from CREDIT_CARD_TRANSACTIONS";
            Statement st = con.createStatement();
            ResultSet rSet = st.executeQuery(query);

            List<Map<String, Object>> result = new ArrayList<>();// JDK7++
            while (rSet.next()) {
                Map<String, Object> row = new HashMap<>();
                row.put("CARD_NUMBER", rSet.getString("CARD_NUMBER"));
                row.put("FIRST_NAME", rSet.getString("FIRST_NAME"));
                row.put("OPENING_BALANCE", rSet.getString("OPENING_BALANCE"));
                row.put("WITHDRAWAL", rSet.getString("WITHDRAWAL"));
                row.put("DEPOSIT", rSet.getString("DEPOSIT"));
                row.put("CLOSING_BAL", rSet.getString("CLOSING_BAL"));
                row.put("TXDATE", rSet.getString("TXDATE"));
                row.put("USAGE_TYPE", rSet.getString("USAGE_TYPE"));
                result.add(row);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

        ObjectMapper mapper = new ObjectMapper();

        try {
            response.getWriter().write(mapper.writeValueAsString(response));
            response.getWriter().flush();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
    }

}

下面是堆栈跟踪：

com.fasterxml.jackson.databind.JsonMappingException: getWriter() has already been called for this response (through reference chain: org.apache.catalina.connector.ResponseFacade["outputStream"])
    at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:210)
    at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:177)
    at com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:190)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:671)
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:156)
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:129)
    at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:3383)
    at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:2777)
    at com.In10s.getTable.GetTable.doGet(GetTable.java:64)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:620)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:504)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:421)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1074)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
    at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2466)
    at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2455)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:722)
Caused by: java.lang.IllegalStateException: getWriter() has already been called for this response
    at org.apache.catalina.connector.Response.getOutputStream(Response.java:644)
    at org.apache.catalina.connector.ResponseFacade.getOutputStream(ResponseFacade.java:196)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:536)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:663)
    ... 28 more

Answer 1

您正在尝试将HttpServletResponse转换为JSON，并将其发送到HttpServletResponse。您希望将result列表转换为JSON：

response.getWriter().write(mapper.writeValueAsString(result));