playframework 数据库管理工具 笔记（3）

本文将从源代码的层面解析play自带的数据库管理插件。

先看程序的入口：

 @Override
    public void beforeInvocation() {
        if(disabled || Play.mode.isProd()) {
            return;
        }
        try {
            checkEvolutionsState();
        } catch (InvalidDatabaseRevision e) {
            if ("mem".equals(Play.configuration.getProperty("db")) && listDatabaseEvolutions().peek().revision == 0) {
                Logger.info("Automatically applying evolutions in in-memory database");
                applyScript(true);
            } else {
                throw e;
            }
        }
    }

如果当前是生产环境（prod）直接return，如果是开发环境，会进入checkEvolutionsState()

 public synchronized static void checkEvolutionsState() {
        if (getDatasource() != null && evolutionsDirectory.exists()) {
            List<Evolution> evolutionScript = getEvolutionScript();
            Connection connection = null;
            try {
                connection = getNewConnection();
                ResultSet rs = connection.createStatement().executeQuery("select id, hash, apply_script, revert_script, state, last_problem from play_evolutions where state like 'applying_%'");
                if (rs.next()) {
                    int revision = rs.getInt("id");
                    String state = rs.getString("state");
                    String hash = rs.getString("hash").substring(0, 7);
                    String script = "";
                    if (state.equals("applying_up")) {
                        script = rs.getString("apply_script");
                    } else {
                        script = rs.getString("revert_script");
                    }
                    script = "# --- Rev:" + revision + "," + (state.equals("applying_up") ? "Ups" : "Downs") + " - " + hash + "\n\n" + script;
                    String error = rs.getString("last_problem");
                    throw new InconsistentDatabase(script, error, revision);
                }
            } catch (SQLException e) {
                throw new UnexpectedException(e);
            } finally {
                closeConnection(connection);
            }

            if (!evolutionScript.isEmpty()) {
                throw new InvalidDatabaseRevision(toHumanReadableScript(evolutionScript));
            }
        }
    }

仔细阅读源码会发现，第一句话的意思是：如果application.config中有db.xxx存在，且工程下有”db/evolutions“目录，程序将往下执行getEvolutionScript()方法，现在我们进入这个方法：

 public synchronized static List<Evolution> getEvolutionScript() {
        Stack<Evolution> app = listApplicationEvolutions();
        Stack<Evolution> db = listDatabaseEvolutions();
        List<Evolution> downs = new ArrayList<Evolution>();
        List<Evolution> ups = new ArrayList<Evolution>();

        // Apply non conflicting evolutions (ups and downs)
        while (db.peek().revision != app.peek().revision) {
            if (db.peek().revision > app.peek().revision) {
                downs.add(db.pop());
            } else {
                ups.add(app.pop());
            }
        }

        // Revert conflicting to fork node
        while (db.peek().revision == app.peek().revision && !(db.peek().hash.equals(app.peek().hash))) {
            downs.add(db.pop());
            ups.add(app.pop());
        }

        // Ups need to be applied earlier first
        Collections.reverse(ups);

        List<Evolution> script = new ArrayList<Evolution>();
        script.addAll(downs);
        script.addAll(ups);

        return script;
    }

其中 listApplicationEvolutions()是读取”db/evolutions“目录下的n.sql文件，把相关信息排序以后放入一个Stack<Evolution>里面。

         listDatabaseEvolutions()大致功能是创建”play_evolutions“表，如果这个表存在把表内容读入到Stack<Evolution>里面。

前面的stack取名为app，后面的stack取名为db，分别代表当前工程”db/evolutions“目录下的sql脚本信息和"play_evolutions"表信息

当aap的版本和db的版本不一致时的处理方式：

  while (db.peek().revision != app.peek().revision) {
            if (db.peek().revision > app.peek().revision) {
                downs.add(db.pop());
            } else {
                ups.add(app.pop());
            }
        }

当版本一致但是hash不一致时的处理方式：

 // Revert conflicting to fork node
        while (db.peek().revision == app.peek().revision && !(db.peek().hash.equals(app.peek().hash))) {
            downs.add(db.pop());
            ups.add(app.pop());
        }

当程序走完beforeInvocation()后会以抛出异常的形式把存入表"play_evolutions"的相关sql显示在页面，当按下“Apply_evolutions”的时候 会执行

 @Override
    public boolean rawInvocation(Request request, Response response) throws Exception {

        // Mark an evolution as resolved
        if (Play.mode.isDev() && request.method.equals("POST") && request.url.matches("^/@evolutions/force/[0-9]+$")) {
            int revision = Integer.parseInt(request.url.substring(request.url.lastIndexOf("/") + 1));
            resolve(revision);
            new Redirect("/").apply(request, response);
            return true;
        }

        // Apply the current evolution script
        if (Play.mode.isDev() && request.method.equals("POST") && request.url.equals("/@evolutions/apply")) {
            applyScript(true);
            new Redirect("/").apply(request, response);
            return true;
        }
        return super.rawInvocation(request, response);
    }

这里包含了执行回滚sql的判断和执行正常sql的判断，进入applyScript(true)

public static synchronized boolean applyScript(boolean runScript) {
        try {
            Connection connection = getNewConnection();
            int applying = -1;
            try {
                for (Evolution evolution : getEvolutionScript()) {
                    applying = evolution.revision;

                    // Insert into logs
                    if (evolution.applyUp) {
                        PreparedStatement ps = connection.prepareStatement("insert into play_evolutions values(?, ?, ?, ?, ?, ?, ?)");
                        ps.setInt(1, evolution.revision);
                        ps.setString(2, evolution.hash);
                        ps.setDate(3, new Date(System.currentTimeMillis()));
                        ps.setString(4, evolution.sql_up);
                        ps.setString(5, evolution.sql_down);
                        ps.setString(6, "applying_up");
                        ps.setString(7, "");
                        ps.execute();
                    } else {
                        execute("update play_evolutions set state = 'applying_down' where id = " + evolution.revision);
                    }
                    // Execute script
                    if (runScript) {
                       for (CharSequence sql : new SQLSplitter((evolution.applyUp ? evolution.sql_up : evolution.sql_down))) {
                            final String s = sql.toString().trim();
                            if (StringUtils.isEmpty(s)) {
                                continue;
                            }
                            execute(s);
                        }
                    }
                    // Insert into logs
                    if (evolution.applyUp) {
                        execute("update play_evolutions set state = 'applied' where id = " + evolution.revision);
                    } else {
                        execute("delete from play_evolutions where id = " + evolution.revision);
                    }
                }
                return true;
            } catch (Exception e) {
                String message = e.getMessage();
                if (e instanceof SQLException) {
                    SQLException ex = (SQLException) e;
                    message += " [ERROR:" + ex.getErrorCode() + ", SQLSTATE:" + ex.getSQLState() + "]";
                }
                PreparedStatement ps = connection.prepareStatement("update play_evolutions set last_problem = ? where id = ?");
                ps.setString(1, message);
                ps.setInt(2, applying);
                ps.execute();
                closeConnection(connection);
                Logger.error(e, "Can't apply evolution");
                return false;
            }
        } catch (Exception e) {
            throw new UnexpectedException(e);
        }
    }

可以看到这个方法的作用是更新“play_evolution”表，并执行app.up和db.down(如果app.up不存在).

执行完applyScript(true)后，url地址被转发回到当前请求页面，到这里大致的流程就走完了。

总结：我们可以看到，play每次读取的都是application.config中默认的db配置，即db.标注的数据库连接配置，所以同一时间只能做到更新到一台服务器，同时，回滚的方式为执行"# --- !down"标注下的sql，一旦误操作或者删除工程中配置的n.sql回滚以后写入的数据很难恢复。从程序设计角度来看play更鼓励我们在dev模式下使用这个插件(实际上，在生产环境中必须要执行play的脚本命令才能完成相关操作，具体参见“笔记（2）”)。同时程序做到了兼容mysql和oracle，且是线程安全的。