java编程奇淫技巧之让java帮我们生成java代码后不停服，直接编译为class并且反射为对象直接使用-白红宇

java编程奇淫技巧之让java帮我们生成java代码后不停服，直接编译为class并且反射为对象直接使用

阅读量：4072 次

发布时间：2019-05-25

本文共 6913 字，大约阅读时间需要 23 分钟。

项目中总不乏简单的CURD，甚至稍微复杂点的CURD，是不需要写后端代码的，只要抽象的好，完全可以一个base类全部搞定，前端代码也可以使用 js来做简单的配置直接生成见：

我们可以让java去解析js的列配置来拼接sql，注册到mybatis中，然后执行，返回map，但是遇到一个问题，我项目中有一个服务是必须传对象才能处理，因此我需要类对象，而不是map或者json，但是我是通过JS来配置CRUD的并么有写一行代码怎么办呢？？？？

于是找动态生成java的解决方案。

首先看到了此篇文章：介绍了由复杂到简单的各种方案，最终决定用最简单的方案来冬天根据配置生成java代码 - > 编译javaclass->使用自定义类加载器加载class -> 遍历数据库读取的内容->反射通过class得到对象->调用系统服务把这堆对象传进去做数据处理->把对象转换为json。

上干货，我已经写好了，直接运行main方法即可，你需要自己写一个java 文件然后读取他放到我提供的方法中然后就可以得到class对象了。

package com.ylm.core.clazz;import com.ylm.common.utils.FileUtils;import com.ylm.common.utils.ReflectUtils;import com.ylm.core.base.bean.SuperBean;import javax.tools.*;import java.io.*;import java.net.URI;import java.net.URL;import java.net.URLClassLoader;import java.nio.CharBuffer;import java.util.*;import javax.tools.JavaFileObject.Kind;/** *  把一段Java字符串变成类 * @ProjectName: framework_v2_idea2 * @Package: com.ylm.core.clazz * @ClassName: MemoryClassLoader * @Author: JackWang * @CreateDate: 2018/12/5 0005 20:26 * @UpdateUser: JackWang * @UpdateDate: 2018/12/5 0005 20:26 * @Version: 1.0 */public class MemoryClassLoader extends URLClassLoader {    private Map
   
     classBytes = new HashMap
    
     ();    /**     * 单利默认的     */    private static final MemoryClassLoader defaultLoader = new MemoryClassLoader();    private MemoryClassLoader(){ super(new URL[0], MemoryClassLoader.class.getClassLoader());}    /**     * 获取默认的类加载器     * @return 类加载器对象     */    public static  MemoryClassLoader getInstrance(){        return defaultLoader;    }    /**     * 注册Java 字符串到内存类加载器中     * @param className 类名字     * @param javaStr Java字符串     */    public void registerJava(String className,String javaStr)    {        this.classBytes.putAll(compile( className,  javaStr) );    }    private static Map
     
       compile(String className, String javaStr) {        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();        StandardJavaFileManager stdManager = compiler.getStandardFileManager(null, null, null);        try (MemoryJavaFileManager manager = new MemoryJavaFileManager(stdManager)) {            JavaFileObject javaFileObject = manager.makeStringSource(className, javaStr);            JavaCompiler.CompilationTask task = compiler.getTask(null, manager, null, null, null, Arrays.asList(javaFileObject));            if (task.call())                return manager.getClassBytes();        } catch (IOException e) {            e.printStackTrace();        }        return null;    }  /*  public static void main(String[] args) throws FileNotFoundException, ClassNotFoundException, IllegalAccessException, InstantiationException {        *//*JavacProcessingEnvironment n = null;*//*        MemoryClassLoader loader = MemoryClassLoader.getInstrance();        loader.registerJava("Test", FileUtils.readTxtFile(new FileInputStream(new File("d:/Test.java"))));        Class testClass = loader.findClass("Test");        Object obj = testClass.newInstance();        SuperBean
       superb = (SuperBean)obj;        superb.getTransMap().put("hello","world");        System.out.println(superb.getTransMap());    }*/    @Override    protected Class
       findClass(String name) throws ClassNotFoundException {        byte[] buf = classBytes.get(name);        if (buf == null) {            return super.findClass(name);        }        classBytes.remove(name);        return defineClass(name, buf, 0, buf.length);    }}/** *  内存Java文件管理器 */class MemoryJavaFileManager extends ForwardingJavaFileManager
      
        {    // compiled classes in bytes:    final Map
       
         classBytes = new HashMap
        
         (); final Map
         
          > classObjectPackageMap = new HashMap<>(); MemoryJavaFileManager(JavaFileManager fileManager) { super(fileManager); } public Map
          
            getClassBytes() { return new HashMap
           
            (this.classBytes); } @Override public void flush() throws IOException { } @Override public void close() throws IOException { classBytes.clear(); } @Override public Iterable
            
              list(Location location, String packageName, Set
             
               kinds, boolean recurse) throws IOException { Iterable
              
                it = super.list(location, packageName, kinds, recurse); if (kinds.contains(Kind.CLASS)) { final List
               
                 javaFileObjectList = classObjectPackageMap.get(packageName); if (javaFileObjectList != null) { if (it != null) { for (JavaFileObject javaFileObject : it) { javaFileObjectList.add(javaFileObject); } } return javaFileObjectList; } else { return it; } } else { return it; } } @Override public String inferBinaryName(Location location, JavaFileObject file) { if (file instanceof MemoryInputJavaClassObject) { return ((MemoryInputJavaClassObject)file).inferBinaryName(); } return super.inferBinaryName(location, file); } @Override public JavaFileObject getJavaFileForOutput(JavaFileManager.Location location, String className, Kind kind, FileObject sibling) throws IOException { if (kind == Kind.CLASS) { return new MemoryOutputJavaClassObject(className); } else { return super.getJavaFileForOutput(location, className, kind, sibling); } } JavaFileObject makeStringSource(String className, final String code) { String classPath = className.replace('.', '/') + Kind.SOURCE.extension; return new SimpleJavaFileObject(URI.create("string:///" + classPath), Kind.SOURCE) { @Override public CharBuffer getCharContent(boolean ignoreEncodingErrors) { return CharBuffer.wrap(code); } }; } void makeBinaryClass(String className, final byte[] bs) { JavaFileObject javaFileObject = new MemoryInputJavaClassObject(className, bs); String packageName = ""; int pos = className.lastIndexOf('.'); if (pos > 0) { packageName = className.substring(0, pos); } List
                
                  javaFileObjectList = classObjectPackageMap.get(packageName); if (javaFileObjectList == null) { javaFileObjectList = new LinkedList<>(); javaFileObjectList.add(javaFileObject); classObjectPackageMap.put(packageName, javaFileObjectList); } else { javaFileObjectList.add(javaFileObject); } } class MemoryInputJavaClassObject extends SimpleJavaFileObject { final String className; final byte[] bs; MemoryInputJavaClassObject(String className, byte[] bs) { super(URI.create("string:///" + className.replace('.', '/') + Kind.CLASS.extension), Kind.CLASS); this.className = className; this.bs = bs; } @Override public InputStream openInputStream() { return new ByteArrayInputStream(bs); } public String inferBinaryName() { return className; } } class MemoryOutputJavaClassObject extends SimpleJavaFileObject { final String className; MemoryOutputJavaClassObject(String className) { super(URI.create("string:///" + className.replace('.', '/') + Kind.CLASS.extension), Kind.CLASS); this.className = className; } @Override public OutputStream openOutputStream() { return new FilterOutputStream(new ByteArrayOutputStream()) { @Override public void close() throws IOException { out.close(); ByteArrayOutputStream bos = (ByteArrayOutputStream) out; byte[] bs = bos.toByteArray(); classBytes.put(className, bs); makeBinaryClass(className, bs); } }; } }}

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