RESTful HttpServlet 樣本

RESTful 的核心就是制定用戶端與伺服器互動的統一介面，HttpServlet 基於 HTTP ，可以使用 doXXX 方法來定義CRUD方法。

下面這個栗子，使用了HttpServlet 來實現 RESTful 風格的設計，用doGet 定義查詢介面，用doPost 定義建立介面，用 doPut 定義更新介面，用doDelete 定義刪除介面。Resource representation 格式使用 JSON . domain ，一個類，Prediction，who predicts what .

package com.cherry.example.domain;import java.io.Serializable;public class Prediction implements Serializable, Comparable<Prediction> {    private String who; // person    private String what; // his/her prediction    private int id; // identifier used as lookup key    public Prediction() {    }    public void setWho(String who) {        this.who = who;    }    public String getWho() {        return this.who;    }    public void setWhat(String what) {        this.what = what;    }    public String getWhat() {        return this.what;    }    public void setId(int id) {        this.id = id;    }    public int getId() {        return this.id;    }    public int compareTo(Prediction other) {        return this.id - other.id;    }}

repository ，不用資料庫，就用一個Map簡單表述

package com.cherry.example.repository;import java.beans.XMLEncoder;import java.io.BufferedReader;import java.io.ByteArrayOutputStream;import java.io.IOException;import java.io.InputStream;import java.io.InputStreamReader;import java.util.concurrent.ConcurrentHashMap;import java.util.concurrent.ConcurrentMap;import java.util.concurrent.atomic.AtomicInteger;import javax.servlet.ServletContext;import com.cherry.example.domain.Prediction;public class Predictions {    private ConcurrentMap<Integer, Prediction> predictions;    private ServletContext sctx;    private AtomicInteger mapKey;    public Predictions() {        predictions = new ConcurrentHashMap<Integer, Prediction>();        mapKey = new AtomicInteger();    }    public void setServletContext(ServletContext sctx) {        this.sctx = sctx;    }    public ServletContext getServletContext() {        return this.sctx;    }    public void setMap(ConcurrentMap<String, Prediction> predictions) {        // no-op for now    }    public ConcurrentMap<Integer, Prediction> getMap() {        // Has the ServletContext been set?        if (getServletContext() == null)            return null;        // Has the data been read already?        if (predictions.size() < 1)            populate();        return this.predictions;    }       public int addPrediction(Prediction p) {        int id = mapKey.incrementAndGet();        p.setId(id);        predictions.put(id, p);        return id;    }       private void populate() {        String filename = "/WEB-INF/data/predictions.db";        InputStream in = sctx.getResourceAsStream(filename);        // Read the data into the array of Predictions.        if (in != null) {            try {                InputStreamReader isr = new InputStreamReader(in);                BufferedReader reader = new BufferedReader(isr);                String record = null;                while ((record = reader.readLine()) != null) {                    String[] parts = record.split("!");                    Prediction p = new Prediction();                    p.setWho(parts[0]);                    p.setWhat(parts[1]);                    addPrediction(p);                }            } catch (IOException e) {            }        }    }}

web ，一個 Servlet ，定義了CRUD介面

package com.cherry.example.web;import java.io.BufferedReader;import java.io.InputStreamReader;import java.io.OutputStream;import java.util.Arrays;import java.util.concurrent.ConcurrentMap;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.xml.ws.http.HTTPException;import com.cherry.example.domain.Prediction;import com.cherry.example.repository.Predictions;import com.google.gson.Gson;@WebServlet(urlPatterns = "/predictions")public class PredictionsServlet extends HttpServlet {    private Predictions predictions; // backend bean    // Executed when servlet is first loaded into container.    // Create a Predictions object and set its servletContext    // property so that the object can do I/O.    @Override    public void init() {        predictions = new Predictions();        predictions.setServletContext(this.getServletContext());    }    // GET /predictions    // GET /predictions?id=1    @Override    public void doGet(HttpServletRequest request, HttpServletResponse response) {        String result = null;        Gson gson = new Gson();        String param = request.getParameter("id");        Integer key = (param == null) ? null : new Integer(param.trim());        // If no query string, assume client wants the full list.        if (key == null) {            ConcurrentMap<Integer, Prediction> map = predictions.getMap();            // Sort the map's values for readability.            Object[] list = map.values().toArray();            Arrays.sort(list);            result = gson.toJson(list);        }        // Otherwise, return the specified Prediction.        else {            Prediction pred = predictions.getMap().get(key);            if (pred == null) { // no such Prediction                String msg = key + " does not map to a prediction.\n";                result = gson.toJson(msg);            } else { // requested Prediction found                result = gson.toJson(pred);            }        }        // response        sendResponse(response, result);    }    // POST /predictions    // HTTP body should contain two keys, one for the predictor ("who") and    // another for the prediction ("what").    @Override    public void doPost(HttpServletRequest request, HttpServletResponse response) {        String who = request.getParameter("who");        String what = request.getParameter("what");        // Are the data to create a new prediction present?        if (who == null || what == null)            throw new HTTPException(HttpServletResponse.SC_BAD_REQUEST);        // Create a Prediction.        Prediction p = new Prediction();        p.setWho(who);        p.setWhat(what);        // Save the ID of the newly created Prediction.        int id = predictions.addPrediction(p);        // Generate the confirmation message.        String msg = "Prediction " + id + " created.\n";        sendResponse(response, msg);    }    // PUT /predictions    // HTTP body should contain at least two keys: the prediction's id    // and either who or what.    @Override    public void doPut(HttpServletRequest request, HttpServletResponse response) {        /*         * A workaround is necessary for a PUT request because neither Tomcat nor Jetty         * generates a workable parameter map for this HTTP verb.         */        String key = null;        String rest = null;        boolean who = false;        /* Let the hack begin. */        try {            BufferedReader br = new BufferedReader(new InputStreamReader(request.getInputStream()));            String data = br.readLine();            /*             * To simplify the hack, assume that the PUT request has exactly two parameters:             * the id and either who or what. Assume, further, that the id comes first. From             * the client side, a hash character # separates the id and the who/what, e.g.,             * id=33#who=Homer Allision             */            String[] args = data.split("#"); // id in args[0], rest in args[1]            String[] parts1 = args[0].split("="); // id = parts1[1]            key = parts1[1];            String[] parts2 = args[1].split("="); // parts2[0] is key            if (parts2[0].contains("who"))                who = true;            rest = parts2[1];        } catch (Exception e) {            throw new HTTPException(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);        }        // If no key, then the request is ill formed.        if (key == null)            throw new HTTPException(HttpServletResponse.SC_BAD_REQUEST);        // Look up the specified prediction.        Prediction p = predictions.getMap().get(new Integer(key.trim()));        if (p == null) { // not found?            String msg = key + " does not map to a Prediction.\n";            sendResponse(response, msg);        } else { // found            if (rest == null) {                throw new HTTPException(HttpServletResponse.SC_BAD_REQUEST);            }            // Do the editing.            else {                if (who)                    p.setWho(rest);                else                    p.setWhat(rest);                String msg = "Prediction " + key + " has been edited.\n";                sendResponse(response, msg);            }        }    }    // DELETE /predictions?id=1    @Override    public void doDelete(HttpServletRequest request, HttpServletResponse response) {        String param = request.getParameter("id");        Integer key = (param == null) ? null : new Integer(param.trim());        // Only one Prediction can be deleted at a time.        if (key == null)            throw new HTTPException(HttpServletResponse.SC_BAD_REQUEST);        try {            predictions.getMap().remove(key);            String msg = "Prediction " + key + " removed.\n";            sendResponse(response, msg);        } catch (Exception e) {            throw new HTTPException(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);        }    }    // Method Not Allowed    @Override    public void doTrace(HttpServletRequest request, HttpServletResponse response) {        throw new HTTPException(HttpServletResponse.SC_METHOD_NOT_ALLOWED);    }    @Override    public void doHead(HttpServletRequest request, HttpServletResponse response) {        throw new HTTPException(HttpServletResponse.SC_METHOD_NOT_ALLOWED);    }    @Override    public void doOptions(HttpServletRequest request, HttpServletResponse response) {        throw new HTTPException(HttpServletResponse.SC_METHOD_NOT_ALLOWED);    }    // Send the response payload to the client.    private void sendResponse(HttpServletResponse response, String payload) {        try {            OutputStream out = response.getOutputStream();            out.write(payload.getBytes());            out.flush();        } catch (Exception e) {            throw new HTTPException(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);        }    }}