我谷歌的错误消息getOutputStream() has already been called for this response ，很多人说，这是因为后面的空格或换行<%或%>，但在我的代码，没有一个空格或一个换行符。我在Linux上使用tomcat6。

<%@
    page import="java.servlet.*,
    javax.servlet.http.*,
    java.io.*,
    java.util.*,
    com.lowagie.text.pdf.*,
    com.lowagie.text.*"
    %><%
    response.setContentType("application/pdf");
    Document document = new Document();
    try{
        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
        PdfWriter.getInstance(document, buffer);
        document.open();
        PdfPTable table = new PdfPTable(2);
        table.addCell("1");
        table.addCell("2");
        table.addCell("3");
        table.addCell("4");
        table.addCell("5");
        table.addCell("6");
        document.add(table);
        document.close();
        DataOutput dataOutput = new DataOutputStream(response.getOutputStream());
        byte[] bytes = buffer.toByteArray();
        response.setContentLength(bytes.length);
        for(int i = 0; i < bytes.length; i++)
        {
        dataOutput.writeByte(bytes[i]);
        }
    }catch(DocumentException e){
        e.printStackTrace();
    }

%>
〜

org.apache.jasper.JasperException: java.lang.IllegalStateException: getOutputStream() has already been called for this response
    org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:522)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:410)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717)

根本原因

java.lang.IllegalStateException: getOutputStream() has already been called for this response
    org.apache.catalina.connector.Response.getWriter(Response.java:610)
    org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:198)
    org.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:125)
    org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:118)
    org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:188)
    org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:118)
    org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:77)
    org.apache.jsp.Account.Domain.testPDF_jsp._jspService(testPDF_jsp.java:94)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:374)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717)