Java运行时动态生成类实现过程详解

Enhancer e = new Enhancer();
e.setSuperclass(...);
e.setStrategy(new DefaultGeneratorStrategy() {
  protected ClassGenerator transform(ClassGenerator cg) {
    return new TransformingGenerator(cg,
      new AddPropertyTransformer(new String[]{ "foo" },
          new Class[] { Integer.TYPE }));
  }});
Object obj = e.create();

Map<String, byte[]> results;
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager stdManager = compiler.getStandardFileManager(null, null, null);
try (MemoryJavaFileManager manager = new MemoryJavaFileManager(stdManager)) {
  JavaFileObject javaFileObject = manager.makeStringSource(fileName, source);
  CompilationTask task = compiler.getTask(null, manager, null, null, null, Arrays.asList(javaFileObject));
  if (task.call()) {
    results = manager.getClassBytes();
  }
}

class MemoryClassLoader extends URLClassLoader {

  Map<String, byte[]> classBytes = new HashMap<String, byte[]>();

  public MemoryClassLoader(Map<String, byte[]> classBytes) {
    super(new URL[0], MemoryClassLoader.class.getClassLoader());
    this.classBytes.putAll(classBytes);
  }

  @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);
  }
}

<dependency>
  <groupId>com.itranswarp</groupId>
  <artifactId>compiler</artifactId>
  <version>1.0</version>
</dependency>

public class StringCompiler {
  public static Object run(String source, String...args) throws Exception {
    // 声明类名
    String className = "Main";
    String packageName = "top.fomeiherz";
    // 声明包名：package top.fomeiherz;
    String prefix = String.format("package %s;", packageName);
    // 全类名：top.fomeiherz.Main
    String fullName = String.format("%s.%s", packageName, className);
    
    // 编译器
    JavaStringCompiler compiler = new JavaStringCompiler();
    // 编译：compiler.compile("Main.java", source)
    Map<String, byte[]> results = compiler.compile(className + ".java", prefix + source);
    // 加载内存中byte到Class<?>对象
    Class<?> clazz = compiler.loadClass(fullName, results);
    // 创建实例
    Object instance = clazz.newInstance();
    Method mainMethod = clazz.getMethod("main", String[].class);
    // String[]数组时必须使用Object[]封装
    // 否则会报错：java.lang.IllegalArgumentException: wrong number of arguments
    return mainMethod.invoke(instance, new Object[]{args});
  }
}

public class StringCompilerTest {
  public static void main(String[] args) throws Exception {
    // 传入String类型的代码
    String source = "import java.util.Arrays;public class Main" +
        "{" +
        "public static void main(String[] args) {" +
        "System.out.println(Arrays.toString(args));" +
        "}" +
        "}";
    StringCompiler.run(source, "1", "2");
  }
}

<dependency>
  <groupId>org.codehaus.groovy</groupId>
  <artifactId>groovy-all</artifactId>
  <version>2.4.13</version>
 </dependency>

@Test
  public void testGroovyClasses() throws Exception {
    //groovy提供了一种将字符串文本代码直接转换成Java Class对象的功能
    GroovyClassLoader groovyClassLoader = new GroovyClassLoader();
    //里面的文本是Java代码,但是我们可以看到这是一个字符串我们可以直接生成对应的Class<?>对象,而不需要我们写一个.java文件
    Class<?> clazz = groovyClassLoader.parseClass("package com.xxl.job.core.glue;\n" +
        "\n" +
        "public class Main {\n" +
        "\n" +
        "  public int age = 22;\n" +
        "  \n" +
        "  public void sayHello() {\n" +
        "    System.out.println(\"年龄是:\" + age);\n" +
        "  }\n" +
        "}\n");
    Object obj = clazz.newInstance();
    Method method = clazz.getDeclaredMethod("sayHello");
    method.invoke(obj);

    Object val = method.getDefaultValue();
    System.out.println(val);
  }