Java——动态生成POJO类

package com.java.test.dynamicpojo;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.lang.reflect.Method;
import java.net.URI;
import java.util.Arrays;

import javax.tools.FileObject;
import javax.tools.ForwardingJavaFileManager;
import javax.tools.JavaCompiler;
import javax.tools.JavaFileManager;
import javax.tools.JavaFileObject;
import javax.tools.SimpleJavaFileObject;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;

public class DynamicPOJO {
        private static String classString = "/* hehha */" + "public class Student{                                  "
                + "       private String  studentId;                      "
                + "       public String getStudentId(){                   "
                + "           return \"小明\";                      "
                + "       }                                               "
                + "}                                                    ";

        private static void createStudent() throws Exception {
            JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
            StandardJavaFileManager standardFileManager = compiler.getStandardFileManager(null, null, null);
            ClassJavaFileManager classJavaFileManager = new ClassJavaFileManager(standardFileManager);
            StringObject stringObject = new StringObject(new URI("Student.java"), JavaFileObject.Kind.SOURCE, classString);
            JavaCompiler.CompilationTask task = compiler.getTask(null, classJavaFileManager, null, null, null,
                    Arrays.asList(stringObject));
            if (task.call()) {
                ClassJavaFileObject javaFileObject = classJavaFileManager.getClassJavaFileObject();
                ClassLoader classLoader = new MyClassLoader(javaFileObject);
                Object student = classLoader.loadClass("Student").newInstance();
                System.out.println("student-->"+student);//student对象
                Method getStudetnId = student.getClass().getMethod("getStudentId");
                Object invoke = getStudetnId.invoke(student);
                System.out.println("getStudentId-->"+invoke);
            }
        }

        /**    *自定义fileManager    */ 
        static class ClassJavaFileManager extends ForwardingJavaFileManager{
            private ClassJavaFileObject classJavaFileObject;
            public ClassJavaFileManager(JavaFileManager fileManager) {
                super(fileManager);
            } 
            public ClassJavaFileObject getClassJavaFileObject() { 
                return classJavaFileObject;
            }  
            /**这个方法一定要自定义 */
            @Override  
            public JavaFileObject getJavaFileForOutput(Location location, String className,
                    JavaFileObject.Kind kind, FileObject sibling) throws IOException {
                return (classJavaFileObject = new ClassJavaFileObject(className,kind)); 
            }
        }  
        
        /**     * 存储源文件     */  
        static class StringObject extends SimpleJavaFileObject{
            private String content;
            public StringObject(URI uri, Kind kind, String content) { 
                super(uri, kind);
                this.content = content;
            }   
            @Override 
            public CharSequence getCharContent(boolean ignoreEncodingErrors) throws IOException {
                return this.content;  
            }  
        }   
        
        /**     * class文件（不需要存到文件中）     */
        static class ClassJavaFileObject extends SimpleJavaFileObject{
            ByteArrayOutputStream outputStream; 
            public ClassJavaFileObject(String className, Kind kind) { 
                super(URI.create(className + kind.extension), kind);   
                this.outputStream = new ByteArrayOutputStream(); 
            }        //这个也要实现 
            @Override      
            public OutputStream openOutputStream() throws IOException {  
                return this.outputStream;   
            }       
            public byte[] getBytes(){       
                return this.outputStream.toByteArray();    
                }   
        }   
        
        /**自定义classloader*/
        static class MyClassLoader extends ClassLoader{ 
            private ClassJavaFileObject stringObject;  
            public MyClassLoader(ClassJavaFileObject stringObject){    
                this.stringObject = stringObject;  
            }      
            @Override     
            protected Class<?> findClass(String name) throws ClassNotFoundException {
                byte[] bytes = this.stringObject.getBytes(); 
                return defineClass(name,bytes,0,bytes.length);  
            }  
        }

        
        public static void main(String[] args) throws Exception {
            createStudent();
        }
        
}