如何以编程方式编译和实例化Java类？

参见英文答案 > How do you dynamically compile and load external java classes?                                     2个
我将类名存储在属性文件中.我知道类存储将实现IDynamicLoad.如何动态实例化该类？

现在我有

     Properties foo = new Properties();
    foo.load(new FileInputStream(new File("ClassName.properties")));
    String class_name = foo.getProperty("class","DefaultClass");
    //IDynamicLoad newClass = Class.forName(class_name).newInstance();

newInstance只加载编译的.class文件吗？如何加载未编译的Java类？

解决方法:

How do I load a Java Class that is not compiled?

你需要先编译它.这可以通过javax.tools API以编程方式完成.这只需要将JDK安装在JRE顶部的本地计算机上.

这是一个基本的启动示例(抛开明显的异常处理)：

// Prepare source somehow.
String source = "package test; public class Test { static { System.out.println(\"hello\"); } public Test() { System.out.println(\"world\"); } }";

// Save source in .java file.
File root = new File("/java"); // On Windows running on C:\, this is C:\java.
File sourceFile = new File(root, "test/Test.java");
sourceFile.getParentFile().mkdirs();
Files.write(sourceFile.toPath(), source.getBytes(StandardCharsets.UTF_8));

// Compile source file.
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
compiler.run(null, null, null, sourceFile.getPath());

// Load and instantiate compiled class.
URLClassLoader classLoader = URLClassLoader.newInstance(new URL[] { root.toURI().toURL() });
Class<?> cls = Class.forName("test.Test", true, classLoader); // Should print "hello".
Object instance = cls.newInstance(); // Should print "world".
System.out.println(instance); // Should print "test.Test@hashcode".

产量如何

hello
world
test.Test@ab853b

如果这些类实现了已经在类路径中的某个接口,那么进一步使用会更容易.

SomeInterface instance = (SomeInterface) cls.newInstance();

否则,您需要让Reflection API访问并调用(未知)方法/字段.

这说和实际问题无关：

properties.load(new FileInputStream(new File("ClassName.properties")));

让java.io.File依赖当前工作目录是可移植性问题的秘诀.不要那样做.将该文件放在类路径中,并使用ClassLoader#getResourceAsStream()与类路径相对路径.

properties.load(Thread.currentThread().getContextClassLoader().getResourceAsStream("ClassName.properties"));

标签：java,reflection,dynamic-loading
来源： https://codeday.me/bug/20190911/1803434.html