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发表于 2012-01-19 17:10 |只看该作者 |倒序浏览

groovy 模板引擎实现原理分析

groovy的SimpleTemplateEngine实现了模板功能，类似于jsp。那就分析groovy是如何实现模板的。

使用模板

Java代码

1.Template template = new SimpleTemplateEngine().createTemplate(
2. new StringReader("<% // This is a comment that will be filtered from output %>\n" +
3. "Hello <%out.println(name);%> !")
4. );
5.
6. final StringWriter sw = new StringWriter();
7. template.make([name:'bloodwolf_china').writeTo(sw);
8. println sw.toString();
Template template = new SimpleTemplateEngine().createTemplate(
new StringReader("<% // This is a comment that will be filtered from output %>\n" +
"Hello <%out.println(name);%> !")
);
final StringWriter sw = new StringWriter();
template.make([name:'bloodwolf_china').writeTo(sw);
println sw.toString();

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看看SimpleTemplateEngine类

Java代码

1.public Template createTemplate(Reader reader) throws CompilationFailedException, IOException {
2. SimpleTemplate template = new SimpleTemplate();
3. String script = template.parse(reader);
4.
5. template.script = groovyShell.parse(script, "SimpleTemplateScript" + counter++ + ".groovy");
6.
7. return template;
8. }
public Template createTemplate(Reader reader) throws CompilationFailedException, IOException {
SimpleTemplate template = new SimpleTemplate();
String script = template.parse(reader);
template.script = groovyShell.parse(script, "SimpleTemplateScript" + counter++ + ".groovy");
return template;
}

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这儿做了三件事
1、创建了一个SimpleTemplate对象
2、解析模板，主要是把<%=exp%>转为groovy的内置表达式${exp},把非<%code%>转为调用out.print(内容)函数，<%code%>中的就是groovy代码了。这样就把整个模板解析为一段代码。如
引用
Hello <%out.println(name);%> !
变成

Java代码

1.out.print("Hello ");
2.out.println(name);
3.out.print(" !");
out.print("Hello ");
out.println(name);
out.print(" !");

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3、用groovyShell获取一个Script对象

Script对象只一个支持普通groovy对象，利用了Groovy的特性
实现 getProperty(String property)方法，从参数绑定对象中获取属性，这样脚本中就能获取绑定参数。

Java代码

1.public abstract class Script extends GroovyObjectSupport {
2. private Binding binding;
3. public Object getProperty(String property) {
4. try {
5. return binding.getVariable(property);
6. } catch (MissingPropertyException e) {
7. return super.getProperty(property);
8. }
9. }
10. public abstract Object run();
11.}
public abstract class Script extends GroovyObjectSupport {
private Binding binding;
public Object getProperty(String property) {
try {
return binding.getVariable(property);
} catch (MissingPropertyException e) {
return super.getProperty(property);
}
}
public abstract Object run();
}

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groovyShell把一段代码组装成一个GroovyCodeSource对象,然后调用GroovyClassLoader，CompilationUnit把CodeSource编译成一个Script对象，run()方法中执行的即是out.print(模板内容)这段代码。

在看看如何输出模板内容的

Java代码

1.private static class SimpleTemplate implements Template {
2.
3. protected Script script;
4.
5. public Writable make() {
6. return make(null);
7. }
8.
9. public Writable make(final Map map) {
10. return new Writable() {
11. /**
12. * Write the template document with the set binding applied to the writer.
13. *
14. * @see groovy.lang.Writable#writeTo(java.io.Writer)
15. */
16. public Writer writeTo(Writer writer) {
17. Binding binding;
18. if (map == null)
19. binding = new Binding();
20. else
21. binding = new Binding(map);
22. Script scriptObject = InvokerHelper.createScript(script.getClass(), binding);
23. PrintWriter pw = new PrintWriter(writer);
24. scriptObject.setProperty("out", pw);
25. scriptObject.run();
26. pw.flush();
27. return writer;
28. }
29.
30. /**
31. * Convert the template and binding into a result String.
32. *
33. * @see java.lang.Object#toString()
34. */
35. public String toString() {
36. StringWriter sw = new StringWriter();
37. writeTo(sw);
38. return sw.toString();
39. }
40. };
41. }
42.}
private static class SimpleTemplate implements Template {
protected Script script;
public Writable make() {
return make(null);
}
public Writable make(final Map map) {
return new Writable() {
/**
* Write the template document with the set binding applied to the writer.
*
* @see groovy.lang.Writable#writeTo(java.io.Writer)
*/
public Writer writeTo(Writer writer) {
Binding binding;
if (map == null)
binding = new Binding();
else
binding = new Binding(map);
Script scriptObject = InvokerHelper.createScript(script.getClass(), binding);
PrintWriter pw = new PrintWriter(writer);
scriptObject.setProperty("out", pw);
scriptObject.run();
pw.flush();
return writer;
}
/**
* Convert the template and binding into a result String.
*
* @see java.lang.Object#toString()
*/
public String toString() {
StringWriter sw = new StringWriter();
writeTo(sw);
return sw.toString();
}
};
}
}

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很清楚了，调用make方法，创建一个Script对象，绑定参数binding = new Binding(map)。
创建一个PrintWriter，绑定为out参数，而模板解析的代码中的out.print(内容)就有着落了。

所以：

•Groovy的模板是通过编译，生成Java类，然后调用方法实现的
•使用模板机制注意要缓存Script对象或Template对象，否则每次调用都会编译生成一个新的Java类，导致内存溢出/泄露