Question

我有一个运行数据库查询的JSP页面，并将结果序列化为JSON。我序列化的对象是：

class DataObject {
    private int iTotalRecords;
    private int iTotalDisplayRecords;
    private String sEcho;
    private ArrayList<String> columns;
    private ArrayList<ArrayList<String>> data;
    //getter and setter methods

    public void setColumns( ArrayList<String> cols ) {
        columns = cols;
    }

    public void setTotalDisplayRecords( int _numrecs ) {
        iTotalDisplayRecords = _numrecs;
    }

    public void setTotalRecords( int _numrecs ) {
        iTotalRecords = _numrecs;
    }

    public void setData( ArrayList<ArrayList<String>> _data ) {
        data = _data;
    }

    public void setEcho( String _echo ) {
        sEcho = _echo;
    }

    @Override
    public String toString() {
        return "DataObject [iTotalRecords=" + iTotalRecords + 
            ", iTotalDisplayRecords=" + iTotalDisplayRecords + 
            ", sEcho=" + sEcho + 
            ", columns=" + columns +
            ", data=" + data + "]";
    }

    public DataObject() {
        this.sEcho = "1";
        this.iTotalDisplayRecords = 0;
        this.iTotalRecords = 0;
        this.columns = new ArrayList<String>();
        this.data = new ArrayList<ArrayList<String>>();
    }
}

我简化了页面，JSP（仍然生成空白JSON）如下所示：

String queryOvr = "select 'error' as Input_Parameters";
String sEcho = "0";
LOG.error("db_json_test.jsp - QUERY: " + queryOvr);

int rowCount=0;
int totalCount=0;
Statement stmt = null;
ResultSet returnData = null;
Connection conn = null;
try {
    conn = DatabaseUtilities.getConnection();  //proprietary connection pool of host application
    stmt = conn.createStatement();
    returnData = stmt.executeQuery(queryOvr);
    ResultSetMetaData rsmd = returnData.getMetaData();
    DataObject dObj = new DataObject();

    ArrayList<String> headerRow = new ArrayList<String>();
    int colCount = rsmd.getColumnCount();
    // write column names
    for (int i=0; i<colCount; i++) {
        String headerName = rsmd.getColumnName(i+1);
        if ("".equals(headerName)) {
            headerName = "Column-" + String.valueOf(i+1);
        }
        LOG.error("db_json_test.jsp - HEADER: " + headerName);
        headerRow.add(headerName);
    }
    dObj.setColumns(headerRow);

    // write data
    ArrayList<ArrayList<String>> dataArray = new ArrayList<ArrayList<String>>();
    while ( returnData.next() ) {
        rowCount++;
        ArrayList<String> dataRow = new ArrayList<String>();
        for (int i=0; i<colCount; i++){ 
            String dbData = returnData.getString(i+1);
            LOG.error("db_json_test.jsp - DATA: row: " + headerRow.get(i) + ", value: " + dbData);
            dataRow.add(dbData);
        }
        dataArray.add(dataRow);
        totalCount++;
    }
    dObj.setData(dataArray);

    dObj.setEcho(sEcho);
    dObj.setTotalRecords(totalCount);
    dObj.setTotalDisplayRecords(rowCount);

    LOG.error("db_json_test.jsp - OBJ: " + dObj.toString());
    Type typeResults = new TypeToken<DataObject>(){}.getType();
    Gson gson = new Gson();
    String json = gson.toJson(dObj, typeResults);
    LOG.error("db_json_test.jsp - JSON: " + json);

    //gson.toJson(dObj, typeResults, response.getWriter());
    response.setStatus(HttpServletResponse.SC_OK);
    response.getWriter().write(json);
    response.getWriter().flush();
    response.getWriter().close();
}
catch (Throwable t) {
    LOG.error("db_json.jsp - Error: " + t.toString());
    //t.printStackTrace();

    response.setStatus(HttpServletResponse.SC_OK);
    response.getWriter().write("{\"error\":\"" + t.toString() + "\"}");
    response.getWriter().flush();
    response.getWriter().close();
}

当我查看日志时，我可以看到DataObject实例已正确填充：

783167943 30 Sep 2015 17:37:51,924 ERROR [http-8500-1] JSP            - db_json_test.jsp - QUERY: select 'error' as Input_Parameters
783167959 30 Sep 2015 17:37:51,940 ERROR [http-8500-1] JSP            - db_json_test.jsp - HEADER: Input_Parameters
783167959 30 Sep 2015 17:37:51,940 ERROR [http-8500-1] JSP            - db_json_test.jsp - DATA: row: Input_Parameters, value: error
783167959 30 Sep 2015 17:37:51,940 ERROR [http-8500-1] JSP            - db_json_test.jsp - OBJ: DataObject [iTotalRecords=1, iTotalDisplayRecords=1, sEcho=0, columns=[Input_Parameters], data=[[error]]]
783167959 30 Sep 2015 17:37:51,940 ERROR [http-8500-1] JSP            - db_json_test.jsp - JSON:

然而，当我使用gson.toJson（dObj）进行序列化时，我得到一个空字符串。我已经尝试指定上面示例中显示的类型没有不同的结果。除了我可能不应该在JSP文件中执行此操作的论点之外，我在这里缺少什么？

值得注意的是，catch例程确实将错误消息作为有效的JSON返回，因此在头文件中正确设置了contentType和charset。

以下Java代码生成了预期的JSON ...

public class gsonTest {

    class DataObject {
        private int iTotalRecords;
        private int iTotalDisplayRecords;
        private String sEcho;
        private ArrayList<String> columns;
        private ArrayList<ArrayList<String>> data;

        // getter and setter methods

        public void setColumns(ArrayList<String> cols) {
            columns = cols;
        }

        public void setTotalDisplayRecords(int _numrecs) {
            iTotalDisplayRecords = _numrecs;
        }

        public void setTotalRecords(int _numrecs) {
            iTotalRecords = _numrecs;
        }

        public void setData(ArrayList<ArrayList<String>> _data) {
            data = _data;
        }

        public void setEcho(String _echo) {
            sEcho = _echo;
        }

        @Override
        public String toString() {
            return "DataObject [iTotalRecords=" + iTotalRecords
                    + ", iTotalDisplayRecords=" + iTotalDisplayRecords
                    + ", sEcho=" + sEcho 
                    + ", columns=" + columns 
                    + ", data=" + data + "]";
        }

        public DataObject() {
            this.sEcho = "";
            this.iTotalDisplayRecords = 0;
            this.iTotalRecords = 0;
            this.columns = new ArrayList<String>();
            this.data = new ArrayList<ArrayList<String>>();
        }
    }

    public static void main(String[] args) {
        int totalCount = 0;
        String sEcho = "1";
        Gson gson = new Gson();
        gsonTest tst = new gsonTest();

        Type typeResults = new TypeToken<DataObject>(){}.getType();

        DataObject obj = tst.new DataObject();

        ArrayList<String> headerRow = new ArrayList<String>();
        // write column names
        int colCount = 2;
        for (int i = 0; i < colCount; i++) {
            headerRow.add("Column-" + String.valueOf(i));
        }
        obj.setColumns(headerRow);

        // write data
        ArrayList<ArrayList<String>> dataArray = new ArrayList<ArrayList<String>>();
        int rowCount = 0;
        while (rowCount < 20) {
            ArrayList<String> dataRow = new ArrayList<String>();
            for (int i = 0; i < colCount; i++) {
                dataRow.add("Row-" + String.valueOf(rowCount) + ":Col-"
                        + String.valueOf(i));
            }
            rowCount++;
            dataArray.add(dataRow);
            totalCount++;
        }
        obj.setData(dataArray);

        obj.setEcho(sEcho);
        obj.setTotalRecords(totalCount);
        obj.setTotalDisplayRecords(rowCount);
        String output = gson.toJson(obj,typeResults);
        System.out.println(obj.toString());
        System.out.println(output);
    }
}

这是输出：

DataObject [iTotalRecords = 20，iTotalDisplayRecords = 20，sEcho = 1， columns = [Column-0，Column-1]，data = [[Row-0：Col-0，Row-0：Col-1]， [Row-1：Col-0，Row-1：Col-1]，[Row-2：Col-0，Row-2：Col-1]，[Row-3：Col-0，第3行：第1行]，[第4行：第0列，第4行：第1列]，[第5行：第0列，第5行：第1列]， [Row-6：Col-0，Row-6：Col-1]，[Row-7：Col-0，Row-7：Col-1]，[Row-8：Col-0，第8行：第1行]，[第9行：第0行，第9行：第1列]，[第10行：第0行，第10行：第1行]，[第11行：第0行，第11行：第1列]，[第12行：第0行，第12行：第1行]，[第13行：第0列，第13行：第1列]，[第14行：第0行，第14行：第1行]，[第15行：第0行，第15行：第1列]，[第16行：第0行，第16行：第1行]，[第17行：第0行，第17行：第1列]，[第18行：第0行，第18行：第1栏]，[第19行：第0栏，第19行：第1栏]] {＆＃34; iTotalRecords＆＃34;：20，＆＃34; iTotalDisplayRecords＆＃34;：20，＆＃34; sEcho＆＃34;：＆＃34; 1＆＃34;＆＃34;列＆＃34 ;: [＆＃34;柱-0＆＃34;＆＃34;色谱柱-1＆＃34]，＆＃34;数据＆＃34;：[[＆＃34;行-0：Col-0中＆＃34 ;, ＆＃34;行-0：的Col-1＆＃34;]，[＆＃34;行-1：Col-0中＆＃34;＆＃34;行-1：的Col-1＆＃34;]，[＆＃34;行-2：Col-0中＆＃34;＆＃34;行-2：山口-1＆＃34;]，[＆＃34;行-3：Col-0中＆＃34;＆＃34;行3：的Col-1＆＃34;]，[＆＃34;行4：Col-0中＆＃34;＆＃34;行4：的Col-1＆＃34;]，[＆＃34;行-5：Col-0中＆＃34;＆＃34;行-5：的Col-1＆＃34;]，[＆＃34;行-6：Col-0中＆＃34;＆＃34;行-6： COL-1＆＃34;]，[＆＃34;行-7：Col-0中＆＃34;＆＃34;行-7：COL-1＆＃34;]，[＆＃34;行-8：山口-0＆＃34;＆＃34;行-8：的Col-1＆＃34;]，[＆＃34;行-9：Col-0中＆＃34;＆＃34;行-9：的Col-1＆＃ 34]，[＆＃34;行-10：Col-0中＆＃34;＆＃34;行-10：的Col-1＆＃34;]，[＆＃34;行-11：Col-0中＆＃34 ;，＆＃34;行-11：的Col-1＆＃34;]，[＆＃34;行-12：Col-0中＆＃34;＆＃34;行-12：的Col-1＆＃34]， [＆＃34;行-13：Col-0中＆＃34;＆＃34;行-13：的Col-1＆＃34;]，[＆＃34;行-14：Col-0中＆＃34;＆＃ 34;行-14：的Col-1＆＃34;]，[＆＃34;行-15：Col-0中＆＃34;＆＃34;行-15：的Col-1＆＃34;]，[＆＃34 ;行16：Col-0中＆＃34;＆＃34;行-16：的Col-1＆＃34;]，[＆＃34;行-17：Col-0中＆＃34;＆＃34;行向17：同事1＆＃34;]，[＆＃34;行-18：Col-0中＆＃34;＆＃34;行-18：的Col-1＆＃34;]，[＆＃34;行-19：Col-0中与＃34;＆＃34;行-19：的Col-1＆＃34;]]}

Answer 1

进一步挖掘后，我找到了一个可行的解决方案。通过将测试Java类组合到JSP中，我得到了正确的输出。我不确定为什么这有效，原来没有

<%@ page
   language="java"
   session="true"
   contentType="application/json; charset=UTF-8"
%>
<%@ page import="com.google.gson.Gson,
                java.lang.reflect.Type,
                com.google.gson.reflect.TypeToken,
                java.util.ArrayList,
                java.sql.*,
                org.apache.log4j.Category,
                com.foo.bar.*;"
%>
<% 
final org.apache.log4j.Category LOG = org.apache.log4j.Category.getInstance ("JSP");

class db_json {

    class DataObject {
        private int iTotalRecords;
        private int iTotalDisplayRecords;
        private String sEcho;
        private ArrayList<String> columns;
        private ArrayList<ArrayList<String>> data;

        // getter and setter methods

        public void setColumns(ArrayList<String> cols) {
            columns = cols;
        }

        public void setTotalDisplayRecords(int _numrecs) {
            iTotalDisplayRecords = _numrecs;
        }

        public void setTotalRecords(int _numrecs) {
            iTotalRecords = _numrecs;
        }

        public void setData(ArrayList<ArrayList<String>> _data) {
            data = _data;
        }

        public void setEcho(String _echo) {
            sEcho = _echo;
        }

        @Override
        public String toString() {
            return "DataObject [iTotalRecords=" + iTotalRecords
                    + ", iTotalDisplayRecords=" + iTotalDisplayRecords
                    + ", sEcho=" + sEcho 
                    + ", columns=" + columns 
                    + ", data=" + data + "]";
        }

        public DataObject() {
            this.sEcho = "";
            this.iTotalDisplayRecords = 0;
            this.iTotalRecords = 0;
            this.columns = new ArrayList<String>();
            this.data = new ArrayList<ArrayList<String>>();
        }
    }

    public String testJson(HttpServletRequest request){
        String queryOvr = "select 'error' as Input_Parameters";
        LOG.debug("db_json3.jsp - QUERY: " + queryOvr);
        Gson gson = new Gson();
        int rowCount=0;
        int totalCount=0;
        Statement stmt = null;
        ResultSet returnData = null;
        Connection conn = null;
        try {
            conn = DatabaseUtilities.getConnection();
                stmt = conn.createStatement();
                returnData = stmt.executeQuery(queryOvr);
                ResultSetMetaData rsmd = returnData.getMetaData();
                DataObject dObj = new DataObject();

                ArrayList<String> headerRow = new ArrayList<String>();
                int colCount = rsmd.getColumnCount();
                // write column names
                for (int i=0; i<colCount; i++) {
                    String headerName = rsmd.getColumnName(i+1);
                    if ("".equals(headerName)) {
                        headerName = "Column-" + String.valueOf(i+1);
                    }
                    LOG.debug("db_json3.jsp - HEADER: " + headerName);
                    headerRow.add(headerName);
                }
                dObj.setColumns(headerRow);

                // write data
                ArrayList<ArrayList<String>> dataArray = new ArrayList<ArrayList<String>>();
                while ( returnData.next() ) {
                    if ( rowCount>=iDisplayStart && iDisplayLength>=0 && rowCount<iDisplayLength ) {
                        rowCount++;
                        ArrayList<String> dataRow = new ArrayList<String>();
                        for (int i=0; i<colCount; i++){ 
                            String dbData = returnData.getString(i+1);
                            LOG.debug("db_json3.jsp - DATA: row: " + headerRow.get(i) + ", value: " + dbData);
                            dataRow.add(dbData);
                        }
                        dataArray.add(dataRow);
                    }
                    totalCount++;
                }
                dObj.setData(dataArray);

                dObj.setEcho(sEcho);
                dObj.setTotalRecords(totalCount);
                dObj.setTotalDisplayRecords(rowCount);

                LOG.debug("db_json3.jsp - OBJ: " + dObj.toString());
                Type typeResults = new TypeToken<DataObject>(){}.getType();
                String json = gson.toJson(dObj, typeResults);
                LOG.debug("db_json3.jsp - JSON: " + json);
                return json;
        }
        catch (Throwable t) {
            LOG.debug("db_json3.jsp - Error: " + t.toString());
            return gson.toJson(t);
        }
    }
}
db_json dbjson = new db_json();
String json = dbjson.testJson();
response.setStatus(HttpServletResponse.SC_OK);
response.getWriter().write(json);
response.getWriter().flush();
response.getWriter().close();
%>

使用gson在JSP页面中序列化POJO时，输出为空。我错过了什么？

1 个答案: