最近一直在思考一个问题,不知道大家有没有什么建议。 我有一个 Web 应用程序,需要添加一个用于报告的部分。它将根据用户 gui 输入构建一个 t-sql 查询,执行并允许用户通过浏览器将文件作为 csv 下载到他们的计算机上。就其本身而言,这些都不是太复杂。我以前做过这些事情。然而,有一个问题我以前没有解决过。结果可能最终是值(value)多个 GB 的数据。我会采取哪些与正常情况不同的做法(就代码而言)以防止浏览器过载或速度极慢?显然不能一次性全部保存在内存中吗?我该如何防止这种情况发生?
最佳答案
您可以将 csv 数据直接写入传入请求的 OutputStream:
package example;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.Writer;
import java.nio.charset.StandardCharsets;
public class MyServlet extends HttpServlet {
private final CSVCreator csvCreator;
private MyServlet(CSVCreator csvCreator) {
this.csvCreator = csvCreator;
}
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// be nice to your consumer, tell them what they get
resp.setHeader("Content-Type", "text/comma-separated-values; charset=UTF-8");
try (Writer writer = new OutputStreamWriter(resp.getOutputStream(), StandardCharsets.UTF_8)) {
this.csvCreator.createCSV("some params you need to create the correct csv", writer);
}
}
}
您必须更改 csv 生成才能写入写入器...例如:
package example;
import java.io.IOException;
public class CSVCreator {
public void createCSV(String someParamsYouNeedToCreateTheCSV, Appendable appendable) throws IOException {
appendable.append("Column1,Column2,Column3\n");
appendable.append("Row1,Row1,Row1\n");
appendable.append("Row2,Row2,Row2\n");
}
}
例如,您还可以使用 apache-commons-csv 将 csv 写入给定的 Appendable(请参阅: https://javadoc.io/doc/org.apache.commons/commons-csv/latest/org/apache/commons/csv/CSVPrinter.html )
package example;
import org.apache.commons.csv.CSVFormat;
import org.apache.commons.csv.CSVPrinter;
import java.io.IOException;
public class CSVCreator {
public void createCSV(String someParamsYouNeedToCreateTheCSV, Appendable appendable) throws IOException {
try (CSVPrinter printer = new CSVPrinter(appendable, CSVFormat.DEFAULT)) {
printer.printRecord("Column1", "Column2", "Column3");
printer.printRecord("Row1", "Row1", "Row1");
printer.printRecord("Row2", "Row2", "Row2");
}
}
}
编辑:
您可以从结果集中创建 CSV 文件,如下所示:
package example;
import org.apache.commons.csv.CSVFormat;
import org.apache.commons.csv.CSVPrinter;
import java.io.IOException;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
public class ResultSetToCSV {
public void createCSV(ResultSet rs, Appendable appendable) throws IOException, SQLException {
final ResultSetMetaData metaData = rs.getMetaData();
final int columnCount = metaData.getColumnCount();
try (CSVPrinter printer = new CSVPrinter(appendable, CSVFormat.DEFAULT)) {
printHeaders(metaData, columnCount, printer);
printValues(rs, columnCount, printer);
}
}
private void printHeaders(ResultSetMetaData metaData, int columnCount, CSVPrinter printer) throws IOException, SQLException {
for (int i = 1; i <= columnCount; i++) {
printer.print(metaData.getColumnLabel(i));
}
printer.println();
}
private void printValues(ResultSet rs, int columnCount, CSVPrinter printer) throws IOException, SQLException {
while (rs.next()) {
printValue(rs, columnCount, printer);
}
}
private void printValue(ResultSet rs, int columnCount, CSVPrinter printer) throws IOException, SQLException {
for (int i = 1; i <= columnCount; i++) {
printer.print(rs.getString(i));
}
printer.println();
}
}
关于java - 通过 Java Web 应用程序从查询中下载非常大的结果,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/60515873/