Java：从字符串加载类 [英] Java: Load class from string

问题描述

我知道这可能与类加载器有关，但是我找不到一个例子（可能是我正在搜索错误的关键字。

I know this has probably something to do with class loaders, however I couldn't find an example (it might be I'm google-ing for the wrong keywords.

我正在尝试从字符串中加载一个类（或方法）。该字符串不包含类的名称，而是包含类的代码，例如

I am trying to load a class (or a method) form a string. The string doesn't contain the name of a class, but the code for a class, e.g.

class MyClass implements IMath {
    public int add(int x, int y) {
         return x + y;
    }
}

然后执行以下操作：

String s = "class MyClass implements IMath { public int add(int x, int y) { return x + y; }}";
IMath loadedClass = someThing.loadAndInitialize(string);
int result = loadedClass.add(5,6);

现在显然， someThing.loadAndInitialize（string） - 部分是我不知道如何实现的部分。这甚至可能吗？或者运行JavaScripts会更容易吗？并以某种方式给变量/ ob jects（比如x和y）？

Now obviously, the someThing.loadAndInitialize(string) - part is the one I don't know how to achieve. Is this even possible? Or would it be easier to run JavaScripts and somehow "give" the variables / objects (like x and y)?

感谢您的任何提示。

推荐答案

使用Java Compiler API。 此处是一篇博文，显示你怎么做。

Use Java Compiler API. Here is a blog post that shows you how to do it.

你可以使用临时文件，因为这需要输入/输出文件，或者你可以创建 JavaFileObject 从字符串中读取源代码。来自 javadoc ：

You can use temporary files for this, as this requires input/output file, or you can create custom implementation of JavaFileObject that reads source from string. From the javadoc:

   /**
    * A file object used to represent source coming from a string.
    */
   public class JavaSourceFromString extends SimpleJavaFileObject {
       /**
        * The source code of this "file".
        */
       final String code;

       /**
        * Constructs a new JavaSourceFromString.
        * @param name the name of the compilation unit represented by this file object
        * @param code the source code for the compilation unit represented by this file object
        */
       JavaSourceFromString(String name, String code) {
           super(URI.create("string:///" + name.replace('.','/') + Kind.SOURCE.extension),
                 Kind.SOURCE);
           this.code = code;
       }

       @Override
       public CharSequence getCharContent(boolean ignoreEncodingErrors) {
           return code;
       }
   }

获得输出文件（已编译） .class file），您可以使用 URLClassLoader 加载它，如下所示：

Once you have the output file (which is a compiled .class file), you can load it using URLClassLoader as follows:

    ClassLoader loader = new URLClassLoader(new URL[] {myClassFile.toURL());
    Class myClass = loader.loadClass("my.package.MyClass");

然后实例化它，使用：

    myClass.newInstance();

或使用构造函数。

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