java实现动态编译并动态加载

Java实现动态编译并动态加载是一种非常强大和灵活的技术。本篇文章将介绍如何实现Java的动态编译和加载，并给出两个示例说明。

动态编译的实现

Java中的动态编译是通过使用Java提供的Compiler API来实现的。在Java中，编译器可以将Java源代码编译成字节码，这些字节码可以直接在Java虚拟机上运行。下面是一些使用Java Compiler API实现动态编译的关键步骤：

创建一个JavaCompiler对象。

java JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();

创建一个DiagnosticCollector对象，用于在编译过程中收集诊断信息。

java DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<>();

创建一个StandardJavaFileManager对象，用于管理Java文件。

java StandardJavaFileManager fileManager = compiler.getStandardFileManager(diagnostics, null, null);

创建一个JavaFileObject对象，用于表示Java源代码。

java JavaFileObject fileObject = new JavaSourceFromString(className, code);

创建一个编译任务（CompileTask），并调用它的call方法来编译Java源代码。

java Iterable<? extends JavaFileObject> compilationUnits = Arrays.asList(fileObject); JavaCompiler.CompilationTask task = compiler.getTask(null, fileManager, diagnostics, options, null, compilationUnits); boolean success = task.call();

完整示例代码：

import javax.tools.JavaCompiler;
import javax.tools.ToolProvider;
import javax.tools.DiagnosticCollector;
import javax.tools.JavaFileObject;
import javax.tools.StandardJavaFileManager;
import javax.tools.JavaCompiler.CompilationTask;
import java.io.IOException;
import java.io.StringWriter;
import java.nio.charset.Charset;
import java.util.Arrays;

public class DynamicCompiler {
    public static void main(String[] args) throws IOException {
        String code = "public class HelloWorld { public static void main(String[] args) { System.out.println(\"Hello, world!\"); } }";
        String className = "HelloWorld";

        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<>();
        StandardJavaFileManager fileManager = compiler.getStandardFileManager(diagnostics, null, Charset.forName("UTF-8"));

        JavaFileObject fileObject = new JavaSourceFromString(className, code);

        Iterable<? extends JavaFileObject> compilationUnits = Arrays.asList(fileObject);
        CompilationTask task = compiler.getTask(null, fileManager, diagnostics, null, null, compilationUnits);
        boolean success = task.call();

        if (!success) {
            StringWriter writer = new StringWriter();
            for (var diagnostic : diagnostics.getDiagnostics()) {
                writer.append(diagnostic.getMessage(null)).append("\n");
            }
            throw new RuntimeException(writer.toString());
        }
    }

    static class JavaSourceFromString implements JavaFileObject {
        private String code;
        private String name;

        public JavaSourceFromString(String name, String code) {
            this.code = code;
            this.name = name;
        }

        @Override
        public Kind getKind() {
            return Kind.SOURCE;
        }

        @Override
        public boolean isNameCompatible(String simpleName, Kind kind) {
            return this.name.equals(simpleName + "." + kind.extension);
        }

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

        @Override
        public String getName() {
            return this.name;
        }
    }
}

动态加载的实现

Java提供了多种方式来动态加载已编译的类。其中，ClassLoader是实现动态加载的核心类。ClassLoader可以在运行时加载指定的类文件，从而实现动态编译和加载的功能。下面是一些实现动态加载的关键步骤：

创建一个ClassLoader对象，用于加载指定的类。

java ClassLoader customClassLoader = new CustomClassLoader();

使用ClassLoader的loadClass方法加载指定的类。如果需要使用这个类，可以使用newInstance方法创建实例。

java Class loadedClass = customClassLoader.loadClass(className); Object instance = loadedClass.newInstance();

完整示例代码：

import java.io.IOException;
import java.io.InputStream;

public class CustomClassLoader extends ClassLoader {
    @Override
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        if (name.startsWith("com.example")) {
            try {
                InputStream is = getClass().getResourceAsStream("/" + name.replace(".", "/") + ".class");
                byte[] bytes = new byte[is.available()];
                is.read(bytes);
                return defineClass(name, bytes, 0, bytes.length);
            } catch (IOException e) {
                throw new ClassNotFoundException(name, e);
            }
        }
        return super.loadClass(name);
    }
}

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class DynamicClassLoader {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException {
        CustomClassLoader customClassLoader = new CustomClassLoader();
        Class loadedClass = customClassLoader.loadClass("com.example.HelloWorld");
        Object instance = loadedClass.newInstance();
        Method method = loadedClass.getMethod("hello");
        method.invoke(instance);
    }
}

示例说明

示例1：动态编译并加载一个HelloWorld程序

在本示例中，我们将动态编译并加载一个HelloWorld程序。首先，我们创建一个Java源代码字符串，然后使用Java Compiler API动态编译这个字符串。当编译完成后，我们使用ClassLoader动态加载生成的类，并使用反射API来调用这个类的方法。

示例代码：

import javax.tools.JavaCompiler;
import javax.tools.ToolProvider;
import javax.tools.DiagnosticCollector;
import javax.tools.JavaFileObject;
import javax.tools.StandardJavaFileManager;
import javax.tools.JavaCompiler.CompilationTask;
import java.io.IOException;
import java.io.StringWriter;
import java.nio.charset.Charset;
import java.util.Arrays;

public class DynamicHelloWorld {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException {
        String code = "public class HelloWorld { public static void hello() { System.out.println(\"Hello, world!\"); } }";
        String className = "HelloWorld";

        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<>();
        StandardJavaFileManager fileManager = compiler.getStandardFileManager(diagnostics, null, Charset.forName("UTF-8"));

        JavaFileObject fileObject = new JavaSourceFromString(className, code);

        Iterable<? extends JavaFileObject> compilationUnits = Arrays.asList(fileObject);
        CompilationTask task = compiler.getTask(null, fileManager, diagnostics, null, null, compilationUnits);
        boolean success = task.call();

        if (!success) {
            StringWriter writer = new StringWriter();
            for (var diagnostic : diagnostics.getDiagnostics()) {
                writer.append(diagnostic.getMessage(null)).append("\n");
            }
            throw new RuntimeException(writer.toString());
        }

        CustomClassLoader customClassLoader = new CustomClassLoader();
        Class loadedClass = customClassLoader.loadClass(className);
        Object instance = loadedClass.newInstance();
        Method method = loadedClass.getMethod("hello");
        method.invoke(instance);
    }

    static class JavaSourceFromString implements JavaFileObject {
        private String code;
        private String name;

        public JavaSourceFromString(String name, String code) {
            this.code = code;
            this.name = name;
        }

        @Override
        public Kind getKind() {
            return Kind.SOURCE;
        }

        @Override
        public boolean isNameCompatible(String simpleName, Kind kind) {
            return this.name.equals(simpleName + "." + kind.extension);
        }

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

        @Override
        public String getName() {
            return this.name;
        }
    }
}

示例2：动态加载外部jar包

在本示例中，我们将动态加载外部jar包。我们首先使用URLClassLoader加载这个jar包，然后使用Class.forName方法来加载其中的某个类，最后使用反射API来调用这个类的某个方法。