groovy 模板引擎實現(xiàn)原理分析

CevenCheng 2012-11-25

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groovy 模板引擎實現(xiàn)原理分析

groovy的SimpleTemplateEngine實現(xiàn)了模板功能，類似于jsp。那就分析groovy是如何實現(xiàn)模板的。

使用模板

Java代碼

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();

復制代碼

看看SimpleTemplateEngine類

Java代碼

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;
}

復制代碼

這兒做了三件事
1、創(chuàng)建了一個SimpleTemplate對象
2、解析模板，主要是把<%=exp%>轉(zhuǎn)為groovy的內(nèi)置表達式${exp},把非<%code%>轉(zhuǎn)為調(diào)用out.print(內(nèi)容)函數(shù)，<%code%>中的就是groovy代碼了。這樣就把整個模板解析為一段代碼。如
引用
Hello <%out.println(name);%> !
變成

Java代碼

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獲取一個Script對象

Script對象只一個支持普通groovy對象，利用了Groovy的特性
實現(xiàn) getProperty(String property)方法，從參數(shù)綁定對象中獲取屬性，這樣腳本中就能獲取綁定參數(shù)。

Java代碼

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把一段代碼組裝成一個GroovyCodeSource對象,然后調(diào)用GroovyClassLoader，CompilationUnit把CodeSource編譯成一個Script對象，run()方法中執(zhí)行的即是out.print(模板內(nèi)容)這段代碼。

在看看如何輸出模板內(nèi)容的

Java代碼

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();
}
};
}
}

復制代碼

很清楚了，調(diào)用make方法，創(chuàng)建一個Script對象，綁定參數(shù)binding = new Binding(map)。
創(chuàng)建一個PrintWriter，綁定為out參數(shù)，而模板解析的代碼中的out.print(內(nèi)容)就有著落了。

所以：

?Groovy的模板是通過編譯，生成Java類，然后調(diào)用方法實現(xiàn)的
?使用模板機制注意要緩存Script對象或Template對象，否則每次調(diào)用都會編譯生成一個新的Java類，導致內(nèi)存溢出/泄露