概念

模板方法：Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure. 在一个操作中定义一个算法骨架，并将某些步骤推迟到子类实现。可以让子类在不改变算法整体结构的情况下，重新定义算法中的某些步骤。
算法可以理解为业务逻辑。
模板方法的作用：

复用：子类都可以复用父类中模板方法定义的流程代码。如 InputStream、OutputStream、Reader、Writer，AbstractList 等。
扩展：更多指框架的扩展性，类似控制反转，可以让用户不修改框架源码的情况下，定制化框架的功能。如 Java Servlet、JUnit 等。

示例
public abstract class AbstractClass {
public final void templateMethod() {
//…
method1();
//…
method2();
//…
}

protected abstract void method1();
protected abstract void method2();
}
public class ConcreteClass extends AbstractClass {
@Override
protected void method1() {}

@Override
protected void method2() {}
}
AbstractClass demo = ConcreteClass();
demo.templateMethod();
模板方法用 final 修饰，避免子类重写；扩展点用 abstract 修饰，强迫子类实现。不过这两点都不是必须的。

源码

InputStream

public abstract class InputStream implements Closeable {

public int read(byte b[], int off, int len) throws IOException {
if (b == null) {
throw new NullPointerException();
} else if (off < 0 || len < 0 || len > b.length - off) {
throw new IndexOutOfBoundsException();
} else if (len == 0) {
return 0;
}
int c = read();
if (c == -1) {
return -1;
}
b[off] = (byte)c;
int i = 1;
try {
for (; i < len ; i++) {
c = read();
if (c == -1) {
break;
}
b[off + i] = (byte)c;
}
} catch (IOException ee) {
}
return i;
}

public abstract int read() throws IOException;
}
public class ByteArrayInputStream extends InputStream {
@Override
public synchronized int read() {
return (pos < count) ? (buf[pos++] & 0xff) : -1;
}
}

AbstractList

public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
boolean modified = false;
for (E e : c) {
add(index++, e);
modified = true;
}
return modified;
}
public void add(int index, E element) {
throw new UnsupportedOperationException();
}

Servlet

用户实现的扩展点：
public class HelloServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
this.doPost(req, resp);
}

@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
resp.getWriter().write(“Hello World.”);
}
}
Servlet 容器收到请求后，会找到相应的 Servlet，并调用它的 service 方法。框架的模板方法：
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException
{
HttpServletRequest request;
HttpServletResponse response;
if (!(req instanceof HttpServletRequest &&
res instanceof HttpServletResponse)) {
throw new ServletException(“non-HTTP request or response”);
}
request = (HttpServletRequest) req;
response = (HttpServletResponse) res;
service(request, response);
}
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String method = req.getMethod();
if (method.equals(METHOD_GET)) {
long lastModified = getLastModified(req);
if (lastModified == -1) {
// servlet doesn’t support if-modified-since, no reason
// to go through further expensive logic
doGet(req, resp);
} else {
long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
if (ifModifiedSince < lastModified) {
// If the servlet mod time is later, call doGet()
// Round down to the nearest second for a proper compare
// A ifModifiedSince of -1 will always be less
maybeSetLastModified(resp, lastModified);
doGet(req, resp);
} else {
resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
}
}
} else if (method.equals(METHOD_HEAD)) {
long lastModified = getLastModified(req);
maybeSetLastModified(resp, lastModified);
doHead(req, resp);
} else if (method.equals(METHOD_POST)) {
doPost(req, resp);
} else if (method.equals(METHOD_PUT)) {
doPut(req, resp);
} else if (method.equals(METHOD_DELETE)) {
doDelete(req, resp);
} else if (method.equals(METHOD_OPTIONS)) {
doOptions(req,resp);
} else if (method.equals(METHOD_TRACE)) {
doTrace(req,resp);
} else {
String errMsg = lStrings.getString(“http.method_not_implemented”);
Object[] errArgs = new Object[1];
errArgs[0] = method;
errMsg = MessageFormat.format(errMsg, errArgs);
resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
}
}

JUnit

JUnit 提供了一些扩展点，如 setUp(), tearDown() 等。
public abstract class TestCase extends Assert implements Test {
public void runBare() throws Throwable {
Throwable exception = null;
setUp();
try {
runTest();
} catch (Throwable running) {
exception = running;
} finally {
try {
tearDown();
} catch (Throwable tearingDown) {
if (exception == null) exception = tearingDown;
}
}
if (exception != null) throw exception;
}

/
Sets up the fixture, for example, open a network connection.
This method is called before a test is executed.
*/
protected void setUp() throws Exception {
}
/
Tears down the fixture, for example, close a network connection.
This method is called after a test is executed.
*/
protected void tearDown() throws Exception {
}
}

Callback

回调与模板模式一样也具有复用和扩展的功能。

原理

A 类事先注册某个函数 F 到 B 类，A 类在调用 B 类的 P 函数时，B 类反过来调用 A 类注册给 B 类的 F 函数，F 就叫做回调函数。
回调不仅可以用在代码设计上，还能用在高层次的架构设计上。比如用户向第三方支付系统发起支付，一般不会等到支付结果返回，而是注册回调函数（URL）给第三方支付系统，支付系统执行成功后，将结果通过回调接口返回给用户。
回调有同步回调和异步回调，同步回调更像模板模式，异步回调更像观察者模式。

示例

public interface ICallback {
void methodToCallback();
}
public class BClass {
public void process(ICallback callback) {
//…
callback.methodToCallback();
//…
}
}
public class AClass {
public static void main(String[] args) {
BClass b = new BClass();
b.process(new ICallback() { //回调对象
@Override
public void methodToCallback() {
System.out.println(“Call back me.”);
}
});
}
}

JdbcTemplate

Spring 提供很多 Template 类，但它们并非基于模板方法实现，而是基于 Callback 实现。
如果直接写 JDBC 代码会很冗余，比如：
public class JdbcDemo {
public User queryUser(long id) {
Connection conn = null;
Statement stmt = null;
try {
//1.加载驱动
Class.forName(“com.mysql.jdbc.Driver”);
conn = DriverManager.getConnection(“jdbc:mysql://localhost:3306/demo”, “xzg”, “xzg”);
//2.创建statement类对象，用来执行SQL语句
stmt = conn.createStatement();
//3.ResultSet类，用来存放获取的结果集
String sql = “select from user where id=” + id;
ResultSet resultSet = stmt.executeQuery(sql);
String eid = null, ename = null, price = null;
while (resultSet.next()) {
User user = new User();
user.setId(resultSet.getLong(“id”));
user.setName(resultSet.getString(“name”));
user.setTelephone(resultSet.getString(“telephone”));
return user;
}
} catch (ClassNotFoundException e) {
// TODO: log…
} catch (SQLException e) {
// TODO: log…
} finally {
if (conn != null)
try {
conn.close();
} catch (SQLException e) {
// TODO: log…
}
if (stmt != null)
try {
stmt.close();
} catch (SQLException e) {
// TODO: log…
}
}
return null;
}
}
加载驱动、创建连接、关闭连接等很多操作与业务无关，这些流程大多可以复用，所以 Spring 提供了 JdbcTemplate，用户代码可以变得非常简洁：
public class JdbcTemplateDemo {
private JdbcTemplate jdbcTemplate;
public User queryUser(long id) {
String sql = “select from user where id=”+id;
return jdbcTemplate.query(sql, new UserRowMapper()).get(0);
}
class UserRowMapper implements RowMapper {
public User mapRow(ResultSet rs, int rowNum) throws SQLException {
User user = new User();
user.setId(rs.getLong(“id”));
user.setName(rs.getString(“name”));
user.setTelephone(rs.getString(“telephone”));
return user;
}
}
}
JdbcTemplate 核心的逻辑如下：
@Override
public List query(String sql, RowMapper rowMapper) throws DataAccessException {
return query(sql, new RowMapperResultSetExtractor(rowMapper));
}
@Override
public T query(final String sql, final ResultSetExtractor rse) throws DataAccessException {
class QueryStatementCallback implements StatementCallback, SqlProvider {
@Override
public T doInStatement(Statement stmt) throws SQLException {
ResultSet rs = null;
try {
rs = stmt.executeQuery(sql);
ResultSet rsToUse = rs;
if (nativeJdbcExtractor != null) {
rsToUse = nativeJdbcExtractor.getNativeResultSet(rs);
}
return rse.extractData(rsToUse);
}
finally {
JdbcUtils.closeResultSet(rs);
}
}
@Override
public String getSql() {
return sql;
}
}
return execute(new QueryStatementCallback());
}
@Override
public T execute(StatementCallback action) throws DataAccessException {
Assert.notNull(action, “Callback object must not be null”);
Connection con = DataSourceUtils.getConnection(getDataSource());
Statement stmt = null;
try {
Connection conToUse = con;
if (this.nativeJdbcExtractor != null &&
this.nativeJdbcExtractor.isNativeConnectionNecessaryForNativeStatements()) {
conToUse = this.nativeJdbcExtractor.getNativeConnection(con);
}
stmt = conToUse.createStatement();
applyStatementSettings(stmt);
Statement stmtToUse = stmt;
if (this.nativeJdbcExtractor != null) {
stmtToUse = this.nativeJdbcExtractor.getNativeStatement(stmt);
}
T result = action.doInStatement(stmtToUse);
handleWarnings(stmt);
return result;
}
catch (SQLException ex) {
// Release Connection early, to avoid potential connection pool deadlock
// in the case when the exception translator hasn’t been initialized yet.
JdbcUtils.closeStatement(stmt);
stmt = null;
DataSourceUtils.releaseConnection(con, getDataSource());
con = null;
throw getExceptionTranslator().translate(“StatementCallback”, getSql(action), ex);
}
finally {
JdbcUtils.closeStatement(stmt);
DataSourceUtils.releaseConnection(con, getDataSource());
}
}

Hook

Callback 更加注重语法机制的描述，Hook 更加侧重应用的描述。Tomcat 和 JVM 都有shutdown hook，以 JVM 为例，当应用程序关闭时，会调用 ApplicationShutdownHooks 的 runHooks 方法：
public class ShutdownHookDemo {
private static class ShutdownHook extends Thread {
public void run() {
System.out.println(“I am called during shutting down.”);
}
}
public static void main(String[] args) {
Runtime.getRuntime().addShutdownHook(new ShutdownHook());
}
}
public class Runtime {
public void addShutdownHook(Thread hook) {
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
sm.checkPermission(new RuntimePermission(“shutdownHooks”));
}
ApplicationShutdownHooks.add(hook);
}
}
class ApplicationShutdownHooks {
/ The set of registered hooks /
private static IdentityHashMap hooks;
static {
hooks = new IdentityHashMap<>();
} catch (IllegalStateException e) {
hooks = null;
}
}
static synchronized void add(Thread hook) {
if(hooks == null)
throw new IllegalStateException(“Shutdown in progress”);
if (hook.isAlive())
throw new IllegalArgumentException(“Hook already running”);
if (hooks.containsKey(hook))
throw new IllegalArgumentException(“Hook previously registered”);
hooks.put(hook, hook);
}
static void runHooks() {
Collection threads;
synchronized(ApplicationShutdownHooks.class) {
threads = hooks.keySet();
hooks = null;
}
for (Thread hook : threads) {
hook.start();
}
for (Thread hook : threads) {
while (true) {
try {
hook.join();
break;
} catch (InterruptedException ignored) {
}
}
}
}
}

比较

同步回调几乎与模板模式一致，都是在一个大的算法骨架中，自由替换其中的个别步骤，起到代码复用和扩展的目的。
实现方式：模板方法基于继承；回调基于组合。
组合优于继承，那么回调相对于模板方法的优势如下：

Java 只支持单继承。
回调可以使用匿名类，不用事先定义类。
如果一个类有多个模板方法，每个类都有对应的抽象方法，那么模板方法需要实现所有的抽象方法；而回调只需要往我们需要的模板方法中注入回调对象即可。

CS

模板方法 Template Method

概念

示例

源码