Java 为什么在一个简单的servlet过滤器中不调用getWriter（）？

我已从以下位置下载UpperCaseFilter和CharResponseWrapper类：

UpperCaseFilter.java

package filters;

import java.io.CharArrayWriter;
import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;

public class UpperCaseFilter implements Filter {
  protected FilterConfig config;

  public void init(FilterConfig config) throws ServletException {
    this.config = config;
  }

  public void destroy() {
  }

  public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
      throws ServletException, IOException {
    ServletResponse newResponse = response;
    System.out.println("UpperCasefilter begin");
    if (request instanceof HttpServletRequest) {
      newResponse = new CharResponseWrapper((HttpServletResponse) response);
    }

    chain.doFilter(request, newResponse);

    System.out.println("UpperCaseFilter end");
    if (newResponse instanceof CharResponseWrapper) {
      String text = newResponse.toString();
      if (text != null) {
        text = text.toUpperCase();
        response.getWriter().write(text);
      }
    }
  }
}

CharResponseWrapper.java

package filters;
import java.io.CharArrayWriter;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;

class CharResponseWrapper extends HttpServletResponseWrapper {
  protected CharArrayWriter charWriter;

  protected PrintWriter writer;

  protected boolean getOutputStreamCalled;

  protected boolean getWriterCalled;

  public CharResponseWrapper(HttpServletResponse response) {
    super(response);

    charWriter = new CharArrayWriter();
  }

  public ServletOutputStream getOutputStream() throws IOException {
    if (getWriterCalled) {
      throw new IllegalStateException("getWriter already called");
    }

    getOutputStreamCalled = true;
    return super.getOutputStream();
  }

  public PrintWriter getWriter() throws IOException {
    if (writer != null) {
      return writer;
    }
    if (getOutputStreamCalled) {
      throw new IllegalStateException("getOutputStream already called");
    }
    getWriterCalled = true;
    writer = new PrintWriter(charWriter);
    return writer;
  }

  public String toString() {
    String s = null;

    if (writer != null) {
      s = charWriter.toString();
    }
    return s;
  }
}

并将web.xml文件修改为：

 <filter>
    <filter-name>Filter1</filter-name>
    <filter-class>filters.UpperCaseFilter</filter-class>
</filter>

<filter-mapping>
<filter-name>Filter1</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>

过滤器1
过滤器
过滤器1
/*

我还从主Apache基金会网站下载了“Apache -ToCAT-60.37”。 当我运行web应用程序时，我可以看到我的System.out.println（）语句执行，因此我知道它正在运行UpperCaseFilter类

但是，当我导航到

http://127.0.0.1:8080

所有字符都不是大写！一切看起来都很正常（一个标准的Tomcat主页）

为了调试它，我决定执行“/catalina.sh jpda start”。我注意到CharWrapperResponse.getWriter（）函数从未被调用，因此从未初始化“writer”对象。因此，当UpperCaseFilter调用CharResponseWrapper.toString（）时，“text”变量最终为null，因为writer为null（即，由于writer=null，无法向text写入任何内容）

---

我无法理解为什么从未调用UpperCaseFilter.getWriter（）

是否存在您确定已调用的其他筛选器？不，这是一个非常简单的示例。我从Apache的网站上下载了tomcat，然后立即测试了上面的两个java类？如果在已部署的WAR中配置，则只会在web应用（*）下调用筛选器UpperCaseFilter.class和CharResponseWrapper.class文件已复制到$TOMCAT_HOME/webapps/ROOT/web-INF/classesI知道UpperCaseFilter正在执行，因为我看到了打印语句。