java动态创建class-两种方式（写入文件和非文件）

首先是写入文件方式：

两个文件的目录结构（忽略其他的文件，哈哈哈）

Test.java在org.myself.pojo包下，生成的class文件在target/class下（重点：1.编译class 2.加载class文件）

    private static void createStudentByFile() throws Exception{
        String studentString = "package org.myself.pojo;public class Test{private String studentId;public String getStudentId(){return this.studentId;}public void setStudentId(String studentId){this.studentId = studentId;}}";
        String fileName = System.getProperty("user.dir") + "/src/main/java/org/myself/pojo/Test.java";
        File file = new File(fileName);
        FileWriter fileWriter = new FileWriter(file);
        fileWriter.write(studentString);
        fileWriter.flush();
        fileWriter.close();


        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        StandardJavaFileManager manager = compiler.getStandardFileManager(null,null,null);
        Iterable javaFileObjects = manager.getJavaFileObjects(fileName);
        String dest = System.getProperty("user.dir") + "/target/classes";

        //options就是指定编译输入目录，与我们命令行写javac -d C://是一样的

        List options = new ArrayList();
        options.add("-d");
        options.add(dest);
        JavaCompiler.CompilationTask task = compiler.getTask(null,manager,null,options,null,javaFileObjects);
        task.call();
        manager.close();
        URL[] urls = new URL[]{new URL("file:/" + System.getProperty("user.dir") + "/target/classes")};

        //加载class时要告诉你的classloader去哪个位置加载class文件

        ClassLoader classLoader = new URLClassLoader(urls);
        Object student = classLoader.loadClass("org.myself.pojo.Test").newInstance();
        logger.info("student===={}",student);

    }

不写入文件，直接从内存中创建class

    public static void main(String[] args) {
		try {
			Object student = createStudent();
			Method setStudentIdMethod = student.getClass().getMethod("setStudentId", Integer.class);
			Method getStudentIdMethod = student.getClass().getMethod("getStudentId");
			if (setStudentIdMethod != null) {
				setStudentIdMethod.invoke(student, new Integer(1101));
				Object studentId = getStudentIdMethod.invoke(student);
				logger.info("设置的studentId为：{}", studentId.toString());
			}
		} catch (URISyntaxException e) {
			e.printStackTrace();
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			e.printStackTrace();
		} catch (InstantiationException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		} catch (NoSuchMethodException e) {
			e.printStackTrace();
		} catch (InvocationTargetException e) {
			e.printStackTrace();
		}
	}

	private static String classString = "/* hehha */" +
			"public class Student{                                  " +
			"       private Integer  studentId;                      " +
			"       public Integer getStudentId(){                   " +
			"           return this.studentId;                      " +
			"       }                                               " +
			"       public void setStudentId(Integer studentId){  " +
			"           this.studentId = studentId;                 " +
			"       }                                               " +
			"}                                                    ";

	private static Object createStudent() throws URISyntaxException, ClassNotFoundException, IllegalAccessException, InstantiationException, IOException, NoSuchMethodException, InvocationTargetException {
		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();
			return student;
//			Method getStudetnId = student.getClass().getMethod("getStudentId");
//			Object invoke = getStudetnId.invoke(student);
//			logger.info("class==={}", student);
		}
		return null;
	}
    /**
    *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);
        }
    }