像Eclipse等java IDE是怎么编译和查找java源代码的呢？

源代码保存

这个无需多说，在编译器写入代码，并保存到文件。这个利用流来实现。

编译为class文件

java提供了JavaCompiler，我们可以通过它来编译java源文件为class文件。

查找class

可以通过Class.forName(fullClassPath)或自定义类加载器来实现。

生成对象，并调用对象方法

通过上面一个查找class，得到Class对象后，可以通过newInstance()或构造器的newInstance()得到对象。然后得到Method，最后调用方法，传入相关参数即可。

示例代码：

public class MyIDE {public static void main(String[] args) throws IOException, ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {// 定义java代码，并保存到文件（Test.java）StringBuilder sb = new StringBuilder();sb.append("package com.tommy.core.test.reflect;\n");sb.append("public class Test {\n");sb.append("    private String name;\n");sb.append("    public Test(String name){\n");sb.append("        this.name = name;\n");sb.append("        System.out.println(\"hello,my name is \" + name);\n");sb.append("    }\n");sb.append("    public String sayHello(String name) {\n");sb.append("        return \"hello,\" + name;\n");sb.append("    }\n");sb.append("}\n");System.out.println(sb.toString());String baseOutputDir = "F:\\output\\classes\\";String baseDir = baseOutputDir + "com\\tommy\\core\\test\\reflect\\";String targetJavaOutputPath = baseDir + "Test.java";// 保存为java文件FileWriter fileWriter = new FileWriter(targetJavaOutputPath);fileWriter.write(sb.toString());fileWriter.flush();fileWriter.close();// 编译为class文件JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();StandardJavaFileManager manager = compiler.getStandardFileManager(null,null,null);List<File> files = new ArrayList<>();files.add(new File(targetJavaOutputPath));Iterable compilationUnits = manager.getJavaFileObjectsFromFiles(files);// 编译// 设置编译选项，配置class文件输出路径Iterable<String> options = Arrays.asList("-d",baseOutputDir);JavaCompiler.CompilationTask task = compiler.getTask(null, manager, null, options, null, compilationUnits);// 执行编译任务task.call();// 通过反射得到对象
//        Class clazz = Class.forName("com.tommy.core.test.reflect.Test");// 使用自定义的类加载器加载classClass clazz = new MyClassLoader(baseOutputDir).loadClass("com.tommy.core.test.reflect.Test");// 得到构造器Constructor constructor = clazz.getConstructor(String.class);// 通过构造器new一个对象Object test = constructor.newInstance("jack.tsing");// 得到sayHello方法Method method = clazz.getMethod("sayHello", String.class);// 调用sayHello方法String result = (String) method.invoke(test, "jack.ma");System.out.println(result);}
}

自定义类加载器代码：

public class MyClassLoader extends ClassLoader {private String baseDir;public MyClassLoader(String baseDir) {this.baseDir = baseDir;}@Overrideprotected Class<?> findClass(String name) throws ClassNotFoundException {String fullClassFilePath = this.baseDir + name.replace("\\.","/") + ".class";File classFilePath = new File(fullClassFilePath);if (classFilePath.exists()) {FileInputStream fileInputStream = null;ByteArrayOutputStream byteArrayOutputStream = null;try {fileInputStream = new FileInputStream(classFilePath);byte[] data = new byte[1024];int len = -1;byteArrayOutputStream = new ByteArrayOutputStream();while ((len = fileInputStream.read(data)) != -1) {byteArrayOutputStream.write(data,0,len);}return defineClass(name,byteArrayOutputStream.toByteArray(),0,byteArrayOutputStream.size());} catch (FileNotFoundException e) {e.printStackTrace();} catch (IOException e) {e.printStackTrace();} finally {if (null != fileInputStream) {try {fileInputStream.close();} catch (IOException e) {e.printStackTrace();}}if (null != byteArrayOutputStream) {try {byteArrayOutputStream.close();} catch (IOException e) {e.printStackTrace();}}}}return super.findClass(name);}
}

java源文件和编译后的class文件如下：

关于类的加载器可以参考：https://tommy88.gitee.io/2018/04/11/java-classloader/