How to use JavaPoet?
一、介绍
JavaPoet项目可以动态的生成Java文件，这是一个很强大和很动态的方法，使用注解的时候假如需要生成新的Java文件就可以通过这个开源项目实现。
二、JavaPoet常用的类
MethodSpec 代表一个构造函数或方法声明。
TypeSpec 代表一个类、借口或者枚举声明。
FieldSpec 代表一个成员变量，一个字段声明。
JavaFile 包含一个顶级类的Java文件。
三、引入开源项目
compile 'com.squareup:javapoet:1.8.0'
也可以在开源项目处找到jar文件，下载下来，引入到项目中。
四、生成一个函数
生成一个件简单的HelloWorld类，有一个静态方法，只是简单的输出“Hello World”字符串。
package com.zhangmiao;import javax.lang.model.element.Modifier;import com.squareup.javapoet.*;import java.io.File;import java.io.IOException;import java.lang.System;public class MainClass { /**
* @param args
* */
public static void main(String[] args) { // TODO Auto-generated method stub
MainClass mainClass = new MainClass();
mainClass.generateHelloWord();
}
private void generateHelloWord(){
MethodSpec main = MethodSpec.methodBuilder("show")
.addModifiers(Modifier.PUBLIC,Modifier.STATIC)
.addStatement("$T.out.println($S)",System.class,"Hello World!")
.build();
TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld")
.addModifiers(Modifier.PUBLIC)
.addMethod(main)
.build();
JavaFile javaFile = JavaFile.builder("com.zhangmiao", helloWorld).build();
File outputFile = new File("src/"); //输出文件
try {
javaFile.writeTo(outputFile);
javaFile.writeTo(System.out);
} catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace();
}
}
}运行结果会显示整个类的内容，运行MainClass之后，选择项目点击右键，运行Refresh之后，就可以在包下看到HelloWorld.java类了。
五、调用HelloWorld类的show方法
package com.zhangmiao;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;public class test { /**
* @param args */
public static void main(String[] args) { // TODO Auto-generated method stub
try {
Class hello = Class.forName("com.zhangmiao.HelloWorld");
hello.newInstance();
Method method = hello.getMethod("show", null);
method.invoke(null, null);
} catch (ClassNotFoundException e) { // TODO Auto-generated catch block
System.out.println("Class Not Found");
} catch (InstantiationException e) { // TODO Auto-generated catch block e.printStackTrace();
} catch (IllegalAccessException e) { // TODO Auto-generated catch block e.printStackTrace();
} catch (NoSuchMethodException e) {
System.out.println("No Such Method");
} catch (SecurityException e) { // TODO Auto-generated catch block e.printStackTrace();
} catch (IllegalArgumentException e) { // TODO Auto-generated catch block e.printStackTrace();
} catch (InvocationTargetException e) { // TODO Auto-generated catch block e.printStackTrace();
}
}
}运行test之后，可以看到运行show方法的结果，打印"Hello World"字符串。
六、注意事项
我的代码是使用MyEcplise写的，jdk的版本需要是jdk1.7，如果不是1.7，代码运行就会报如下错误：
Exception in thread "main" java.lang.UnsupportedClassVersionError: com/squareup/javapoet/MethodSpec : Unsupported major.minor version 51.0 at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClass(ClassLoader.java:621) at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124) at java.net.URLClassLoader.defineClass(URLClassLoader.java:260) at java.net.URLClassLoader.access$000(URLClassLoader.java:56) at java.net.URLClassLoader$1.run(URLClassLoader.java:195) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:188) at java.lang.ClassLoader.loadClass(ClassLoader.java:307) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:252) at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320) at MainClass.main(MainClass.java:13)
这个时候就需要将jdk的版本修改为1.7。
还有一些其他生成类和方法的方法，比如for循环的写法，直接使用addCode()方法生成方法内容，设置返回值、参数等等，这些细节可以参考源码地址的README.md文件，很详细，本文只是简单的使用一下。
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