import javax.tools.SimpleJavaFileObject;
import java.net.URI;

/**
 * 将代码保存在字符串构建器中的 Java 源代码。
 *
 * @description:
 * @author: mabh
 * @create: 2022/3/19 22:33
 **/
public class StringBuilderJavaSource extends SimpleJavaFileObject {
    private StringBuilder code;

    public StringBuilderJavaSource(String name) {
        super(URI.create("string:///" + name.replace(".", "/") + Kind.SOURCE.extension), Kind.SOURCE);
        code = new StringBuilder();
    }

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


    public void append(String str) {
        code.append(str);
        code.append("\n");
    }



}

import java.util.Map;

public class MapClassLoader extends ClassLoader {

    private final Map<String, byte[]> classes;

    public MapClassLoader(Map<String, byte[]> classes) {
        this.classes = classes;
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] classBytes = classes.get(name);
        if (classBytes == null) {
            throw new ClassNotFoundException(name);
        }
        Class<?> cl = defineClass(name, classBytes, 0, classBytes.length);
        if (cl == null) {
            throw new ClassNotFoundException(name);
        }
        return cl;
    }
}

import java.util.Map;

public class MapClassLoader extends ClassLoader {

    private final Map<String, byte[]> classes;

    public MapClassLoader(Map<String, byte[]> classes) {
        this.classes = classes;
    }


    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] classBytes = classes.get(name);
        if (classBytes == null) {
            throw new ClassNotFoundException(name);
        }
        Class<?> cl = defineClass(name, classBytes, 0, classBytes.length);
        if (cl == null) {
            throw new ClassNotFoundException(name);
        }
        return cl;
    }
}

需要一个测试类，我们新建一个接口

/**
 * @description:
 * @author: mabh
 * @create: 2022/3/19 23:15
 **/
public interface IABC {
    void main2();
}

重头戏

import javax.tools.*;
import java.io.IOException;
import java.util.*;

/**
 * @description:
 * @author: mabh
 * @create: 2022/3/19 21:49
 **/
public class Test222{
    public static void main(String[] args) throws Exception {
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();

        List<ByteArrayJavaClass> classFileObjects = new ArrayList<>();

        DiagnosticCollector<JavaFileObject> diagnostic = new DiagnosticCollector<>();
        JavaFileManager fileManager = compiler.getStandardFileManager(diagnostic, null, null);
        fileManager = new ForwardingJavaFileManager<JavaFileManager>(fileManager) {
            @Override
            public JavaFileObject getJavaFileForOutput(Location location, String className, JavaFileObject.Kind kind, FileObject sibling) throws IOException {
                ByteArrayJavaClass arrayJavaClass = new ByteArrayJavaClass(className);
                classFileObjects.add(arrayJavaClass);
                return arrayJavaClass;
            }
        };

        StringBuilderJavaSource javaSource = new StringBuilderJavaSource("ABC");
        javaSource.append("package com.epoch.planning;");
        javaSource.append(" public class ABC implements IABC {");
        javaSource.append("    public void main2() {");
        javaSource.append(" System.out.print(\"我是使用java动态编译的类\");");
        javaSource.append("    }");
        javaSource.append(" }");

        JavaCompiler.CompilationTask compilerTask = compiler.getTask(null, fileManager, diagnostic, null, null, Arrays.asList(javaSource));
        Boolean result = compilerTask.call();

        for (Diagnostic<? extends JavaFileObject> diagnosticDiagnostic : diagnostic.getDiagnostics()) {
            System.out.println(diagnosticDiagnostic.getKind() + ": " + diagnosticDiagnostic.getMessage(null));
        }
        fileManager.close();

        if (!result) {
            List<Diagnostic<? extends JavaFileObject>> diagnostics = diagnostic.getDiagnostics();
            for (Diagnostic<? extends JavaFileObject> diagnostic1 : diagnostics) {
                System.out.println(diagnostic1.toString());
                break;
            }
            System.out.println("compile failed!");
        }

        Map<String, byte[]> byteCodeMap = new HashMap<>();
        for (ByteArrayJavaClass classFileObject : classFileObjects) {
            byteCodeMap.put(classFileObject.getName().substring(1), classFileObject.getBytes());
        }
        MapClassLoader loader = new MapClassLoader(byteCodeMap);
        IABC abc = (IABC) loader.findClass("com.epoch.planning.ABC").newInstance();
        abc.main2();


    }
}

失败了会自动打印日志

修改正确即可正确输出