HttpServletResponseWrapper获取jsp内容

WEB开发中，可能会遇到这样的需求，统一修改页面的某类标签。一种办法是直接修改返回的html页面，但这样的做法太粗糙且无复用性。另一种办法是在控制层修改http的响应内容，那么如何在java代码中获取、修改并返回正确的内容呢。又或者我们只是单纯地想更深入地了解httpServletResponse，请看以下内容。

首先，关于http，我们接触最多的两个接口是HttpServletRequest和HttpServletResponse。记住，它们都是接口。

apache官方文档(http://tomcat.apache.org/tomcat-5.5-doc/servletapi/javax/servlet/http/HttpServletResponse.html)是这样描述HttpServletResponse接口的：
扩展于servletResponse接口，在发送响应时提供http定制服务，比如它包含了访问http消息头和cookies的方法。

ServletResponse
servlet容器创建ServletResponse对象并且将其作为参数传递给servlet的服务方法(doGet, doPost等).
通过调用getOutputStream()返回的ServletOutputStream对象实现二进制数据发送。而字符数据的发送，则通过调用getWriter()返回的PrintWriter对象实现。

而已知的HttpServletResponse实现类为HttpServletResponseWrapper
该类可按照开发者意愿被继承，用以适配servlet响应。该类实现了Wrapper或Decorator模式，方法默认按封装的响应对象访问调用。

由此可见，核心部分在于ServletOutputStream对象的处理，也就是HttpServletResponseWrapper如何适配servlet。
public abstract class ServletOutputStream ServletOutputStream extends java.io.OutputStream
提供了发送二进制数据到客户端的输出流。一个servletoutputstream对象通常通过ServletResponse.getOutputStream()方法取回。
我们看到这是一个servlet容器实现的抽象类，它的子类必须实现java.io.OutputStream.write(int)方法。这句很关键，可以说是本次手动实现响应处理的突破口。

总的思路是，利用HttpServletResponseWrapper封装HttpServletResponse，使 HttpServletResponse采用我们自己定义的输入流(OutputStream)。这样，我们就可以通过这个OutputStream得到 目标jsp页面内容。

public void getContent(ServletRequest req, ServletResponse resp,) throws IOException, ServletException{
		ByteArrayOutputStream os =new ByteArrayOutputStream();
		HttpServletResponseWrapper response = new HttpServletResponseWrapper((HttpServletResponse) resp, os);
		//刷新response的输出流, 在某些情况下, 如果不刷新的话, 会导致部分信息还在缓存中, 而没有写入os中
		response.flush();
		String html = new String(os.toByteArray(), "UTF-8");
		resp.getWriter().print(html);
		
		resp.setContentLength(html.getBytes("UTF-8").length);
		resp.setContentType("text/html;charset=UTF-8");
		resp.getWriter().flush();
		
	}

public class HttpServletResponseWrapper extends
		javax.servlet.http.HttpServletResponseWrapper implements Serializable {

	private static final long serialVersionUID = -6723255025479924073L;

	public HttpServletResponseWrapper(HttpServletResponse response,
			OutputStream outputStream) {
		super(response);
		statusCode = 200;
		this.servletOutputStream = new ServletOutputStreamWrapper(outputStream);
	}

	public ServletOutputStream getOutputStream() {
		return servletOutputStream;
	}

	public void setStatus(int code) {
		statusCode = code;
		super.setStatus(code);
	}

	public void sendError(int i, String string) throws IOException {
		statusCode = i;
		super.sendError(i, string);
	}

	public void sendError(int i) throws IOException {
		statusCode = i;
		super.sendError(i);
	}

	public void sendRedirect(String string) throws IOException {
		statusCode = 302;
		super.sendRedirect(string);
	}

	public void setStatus(int code, String msg) {
		statusCode = code;
		super.setStatus(code);
	}

	public int getStatus() {
		return statusCode;
	}

	public void setContentLength(int length) {
		contentLength = length;
		super.setContentLength(length);
	}

	public int getContentLength() {
		return contentLength;
	}

	public void setContentType(String type) {
		contentType = type;
		super.setContentType(type);
	}

	public String getContentType() {
		return contentType;
	}

	public PrintWriter getWriter() throws IOException {
		if (writer == null)
			writer = new PrintWriter(new OutputStreamWriter(
					servletOutputStream, getCharacterEncoding()), true);
		return writer;
	}

	public void addHeader(String name, String value) {
		String header[] = { name, value };
		headers.add(header);
		super.addHeader(name, value);
	}

	public void setHeader(String name, String value) {
		addHeader(name, value);
	}

	public Collection<String[]> getHeaders() {
		return headers;
	}

	public void addCookie(Cookie cookie) {
		cookies.add(cookie);
		super.addCookie(cookie);
	}

	public Collection<Cookie> getCookies() {
		return cookies;
	}

	public void flushBuffer() throws IOException {
		flush();
		super.flushBuffer();
	}

	public void reset() {
		super.reset();
		cookies.clear();
		headers.clear();
		statusCode = 200;
		contentType = null;
		contentLength = 0;
	}

	public void resetBuffer() {
		super.resetBuffer();
	}

	public void flush() throws IOException {
		if (writer != null)
			writer.flush();
		servletOutputStream.flush();
	}

	public String encodeRedirectUrl(String s) {
		return super.encodeRedirectURL(s);
	}

	public String encodeUrl(String s) {
		return super.encodeURL(s);
	}

	private int statusCode;
	private int contentLength;
	private String contentType;
	private final List<String[]> headers = new ArrayList<String[]>();
	private final List<Cookie> cookies = new ArrayList<Cookie>();
	private ServletOutputStream servletOutputStream;
	private PrintWriter writer;
}

public class ServletOutputStreamWrapper extends ServletOutputStream {
	
	public ServletOutputStreamWrapper(OutputStream stream) {
		this.stream = stream;
	}

	public void write(int b) throws IOException {
		stream.write(b);
	}

	public void write(byte b[]) throws IOException {
		stream.write(b);
	}

	public void write(byte b[], int off, int len) throws IOException {
		stream.write(b, off, len);
	}

	private OutputStream stream;
}