Cannot call sendError() after the response has been committed

出现这个错误，一定是多次response导致的，例如下面这段代码就会出现此错误：

import java.io.Writer;

import javax.servlet.http.HttpServletResponse;

import org.apache.struts2.ServletActionContext;

import com.opensymphony.xwork2.ActionSupport;

public class LoginAction extends ActionSupport {

	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;
	private String userName;
	private String pwd;
	private String verifyCode;
	private String ajax;

	// 错误的写法
	@Override
	public String execute() throws Exception {
		// 通过ajax登录
		if (ajax != null) {
			HttpServletResponse response = ServletActionContext.getResponse();
			Writer writer = response.getWriter();
			writer.write("登录成功!");
			writer.flush();
			writer.close();
		}
		return SUCCESS;
	}

	// 正确写法
	public String login1() throws Exception {
		if (ajax != null) {
			HttpServletResponse response = ServletActionContext.getResponse();
			Writer writer = response.getWriter();
			writer.write("登录成功!");
			writer.flush();
			writer.close();
			return null;
		}
		return SUCCESS;
	}

	// 正确写法
	public String login2() throws Exception {
		if (ajax != null) {
			HttpServletResponse response = ServletActionContext.getResponse();
			Writer writer = response.getWriter();
			writer.write("登录成功!");
			writer.flush();
			writer.close();
		}
		return null;
	}

	public String getUserName() {
		return userName;
	}

	public void setUserName(String userName) {
		this.userName = userName;
	}

	public String getPwd() {
		return pwd;
	}

	public void setPwd(String pwd) {
		this.pwd = pwd;
	}

	public String getVerifyCode() {
		return verifyCode;
	}

	public void setVerifyCode(String verifyCode) {
		this.verifyCode = verifyCode;
	}

	public String getAjax() {
		return ajax;
	}

	public void setAjax(String ajax) {
		this.ajax = ajax;
	}
}

以上为登录测试代码（Struts2），在以上示例中，如果判断为ajax!=null成立，那么一定会报如题所示的错误，原因就是：if子句里已经做了一次response，在writer.close();的时候，本次response已经完成；但是紧接着在return SUCCESS;的时候，相当于又做了一次response，所以就出错了~类似的错误也会出现于以下代码中：

protected void doGet(HttpServletRequest request, HttpServletResponse response) 
				throws ServletException, IOException {
			Writer writer = response.getWriter();
			writer.write("Hello");
			writer.flush();
			writer.close();

			response.sendRedirect("www.baidu.com");
		}

出现本错误后，web前端能够接收到第一次response的内容，不会报错，只会在后台显示出错了

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