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刚来公司就接了一个不发版直接改代码的需求


前言


前几天突然接到一个技术需求,想要做一个功能。前端有一个表单,在页面上可以直接写 java 代码,写完后就能保存到数据库,并且这个代码实时生效。这岂非是不用发版就可以随时改代码了吗?而且有bug也不怕,随时改。



适用场景:代码逻辑需要经常变动的业务。


核心思想



  • 页面改动 java 代码字符串
  • java 代码字符串编译成 class
  • 动态加载到 jvm


实现重点


JDK 提供了一个工具包 javax.tools 让使用者可以用简易的 API 进行编译。



这些工具包的使用步骤:



  1. 获取一个 javax.tools.JavaCompiler 实例。
  2. 基于 Java 文件对象初始化一个编译任务 CompilationTask 实例。
  3. 因为JVM 里面的 Class 是基于 ClassLoader 隔离的,所以编译成功之后可以通过自定义的类加载器加载对应的类实例
  4. 使用反射 API 进行实例化和后续的调用。

1. 代码编译


这一步需要将 java 文件编译成 class,其实平常的开发过程中,我们的代码编译都是由 IDEA、Maven 等工具完成。


内置的 SimpleJavaFileObject 是面向源码文件的,而我们的是源码字符串,所以需要实现 JavaFileObject 接口自定义一个 JavaFileObject。


public class CharSequenceJavaFileObject extends SimpleJavaFileObject {

public static final String CLASS_EXTENSION = ".class";

public static final String JAVA_EXTENSION = ".java";

private static URI fromClassName(String className) {
try {
return new URI(className);
} catch (URISyntaxException e) {
throw new IllegalArgumentException(className, e);
}
}

private ByteArrayOutputStream byteCode;
private final CharSequence sourceCode;

public CharSequenceJavaFileObject(String className, CharSequence sourceCode) {
super(fromClassName(className + JAVA_EXTENSION), Kind.SOURCE);
this.sourceCode = sourceCode;
}

public CharSequenceJavaFileObject(String fullClassName, Kind kind) {
super(fromClassName(fullClassName), kind);
this.sourceCode = null;
}

public CharSequenceJavaFileObject(URI uri, Kind kind) {
super(uri, kind);
this.sourceCode = null;
}

@Override
public CharSequence getCharContent(boolean ignoreEncodingErrors) throws IOException {
return sourceCode;
}

@Override
public InputStream openInputStream() {
return new ByteArrayInputStream(getByteCode());
}

// 注意这个方法是编译结果回调的OutputStream,回调成功后就能通过下面的getByteCode()方法获取目标类编译后的字节码字节数组
@Override
public OutputStream openOutputStream() {
return byteCode = new ByteArrayOutputStream();
}

public byte[] getByteCode() {
return byteCode.toByteArray();
}
}

如果编译成功之后,直接通过 CharSequenceJavaFileObject#getByteCode()方法即可获取目标类编译后的字节码对应的字节数组(二进制内容)


2. 实现 ClassLoader


因为JVM 里面的 Class 是基于 ClassLoader 隔离的,所以编译成功之后得通过自定义的类加载器加载对应的类实例,否则是加载不了的,因为同一个类只会加载一次。


主要关注 findClass 方法


public class JdkDynamicCompileClassLoader extends ClassLoader {

public static final String CLASS_EXTENSION = ".class";

private final static Map<String, JavaFileObject> javaFileObjectMap = new ConcurrentHashMap<>();

public JdkDynamicCompileClassLoader(ClassLoader parentClassLoader) {
super(parentClassLoader);
}


@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
JavaFileObject javaFileObject = javaFileObjectMap.get(name);
if (null != javaFileObject) {
CharSequenceJavaFileObject charSequenceJavaFileObject = (CharSequenceJavaFileObject) javaFileObject;
byte[] byteCode = charSequenceJavaFileObject.getByteCode();
return defineClass(name, byteCode, 0, byteCode.length);
}
return super.findClass(name);
}

@Override
public InputStream getResourceAsStream(String name) {
if (name.endsWith(CLASS_EXTENSION)) {
String qualifiedClassName = name.substring(0, name.length() - CLASS_EXTENSION.length()).replace('/', '.');
CharSequenceJavaFileObject javaFileObject = (CharSequenceJavaFileObject) javaFileObjectMap.get(qualifiedClassName);
if (null != javaFileObject && null != javaFileObject.getByteCode()) {
return new ByteArrayInputStream(javaFileObject.getByteCode());
}
}
return super.getResourceAsStream(name);
}

/**
* 暂时存放编译的源文件对象,key为全类名的别名(非URI模式),如club.throwable.compile.HelloService
*/

void addJavaFileObject(String qualifiedClassName, JavaFileObject javaFileObject) {
javaFileObjectMap.put(qualifiedClassName, javaFileObject);
}

Collection<JavaFileObject> listJavaFileObject() {
return Collections.unmodifiableCollection(javaFileObjectMap.values());
}
}

3. 封装了上面的 ClassLoader 和 JavaFileObject


public class JdkDynamicCompileJavaFileManager extends ForwardingJavaFileManager<JavaFileManager> {

private final JdkDynamicCompileClassLoader classLoader;
private final Map<URI, JavaFileObject> javaFileObjectMap = new ConcurrentHashMap<>();

public JdkDynamicCompileJavaFileManager(JavaFileManager fileManager, JdkDynamicCompileClassLoader classLoader) {
super(fileManager);
this.classLoader = classLoader;
}

private static URI fromLocation(Location location, String packageName, String relativeName) {
try {
return new URI(location.getName() + '/' + packageName + '/' + relativeName);
} catch (URISyntaxException e) {
throw new IllegalArgumentException(e);
}
}

@Override
public FileObject getFileForInput(Location location, String packageName, String relativeName) throws IOException {
JavaFileObject javaFileObject = javaFileObjectMap.get(fromLocation(location, packageName, relativeName));
if (null != javaFileObject) {
return javaFileObject;
}
return super.getFileForInput(location, packageName, relativeName);
}

/**
* 这里是编译器返回的同(源)Java文件对象,替换为CharSequenceJavaFileObject实现
*/

@Override
public JavaFileObject getJavaFileForOutput(Location location, String className, JavaFileObject.Kind kind, FileObject sibling) throws IOException {
JavaFileObject javaFileObject = new CharSequenceJavaFileObject(className, kind);
classLoader.addJavaFileObject(className, javaFileObject);
return javaFileObject;
}

/**
* 这里覆盖原来的类加载器
*/

@Override
public ClassLoader getClassLoader(Location location) {
return classLoader;
}

@Override
public String inferBinaryName(Location location, JavaFileObject file) {
if (file instanceof CharSequenceJavaFileObject) {
return file.getName();
}
return super.inferBinaryName(location, file);
}

@Override
public Iterable<JavaFileObject> list(Location location, String packageName, Set<JavaFileObject.Kind> kinds, boolean recurse) throws IOException {
Iterable<JavaFileObject> superResult = super.list(location, packageName, kinds, recurse);
List<JavaFileObject> result = new ArrayList<>();
// 这里要区分编译的Location以及编译的Kind
if (location == StandardLocation.CLASS_PATH && kinds.contains(JavaFileObject.Kind.CLASS)) {
// .class文件以及classPath下
for (JavaFileObject file : javaFileObjectMap.values()) {
if (file.getKind() == JavaFileObject.Kind.CLASS && file.getName().startsWith(packageName)) {
result.add(file);
}
}
// 这里需要额外添加类加载器加载的所有Java文件对象
result.addAll(classLoader.listJavaFileObject());
} else if (location == StandardLocation.SOURCE_PATH && kinds.contains(JavaFileObject.Kind.SOURCE)) {
// .java文件以及编译路径下
for (JavaFileObject file : javaFileObjectMap.values()) {
if (file.getKind() == JavaFileObject.Kind.SOURCE && file.getName().startsWith(packageName)) {
result.add(file);
}
}
}
for (JavaFileObject javaFileObject : superResult) {
result.add(javaFileObject);
}
return result;
}

/**
* 自定义方法,用于添加和缓存待编译的源文件对象
*/

public void addJavaFileObject(Location location, String packageName, String relativeName, JavaFileObject javaFileObject) {
javaFileObjectMap.put(fromLocation(location, packageName, relativeName), javaFileObject);
}
}

4. 使用 JavaCompiler 编译并反射生成实例对象


public final class JdkCompiler {

static DiagnosticCollector<JavaFileObject> DIAGNOSTIC_COLLECTOR = new DiagnosticCollector<>();

@SuppressWarnings("unchecked")
public static <T> T compile(String packageName,
String className,
String sourceCode) throws Exception {
// 获取系统编译器实例
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
// 设置编译参数
List<String> options = new ArrayList<>();
options.add("-source");
options.add("1.8");
options.add("-target");
options.add("1.8");
// 获取标准的Java文件管理器实例
StandardJavaFileManager manager = compiler.getStandardFileManager(DIAGNOSTIC_COLLECTOR, null, null);
// 初始化自定义类加载器
JdkDynamicCompileClassLoader classLoader = new JdkDynamicCompileClassLoader(Thread.currentThread().getContextClassLoader());

// 初始化自定义Java文件管理器实例
JdkDynamicCompileJavaFileManager fileManager = new JdkDynamicCompileJavaFileManager(manager, classLoader);
String qualifiedName = packageName + "." + className;
// 构建Java源文件实例
CharSequenceJavaFileObject javaFileObject = new CharSequenceJavaFileObject(className, sourceCode);
// 添加Java源文件实例到自定义Java文件管理器实例中
fileManager.addJavaFileObject(
StandardLocation.SOURCE_PATH,
packageName,
className + CharSequenceJavaFileObject.JAVA_EXTENSION,
javaFileObject
);
// 初始化一个编译任务实例
JavaCompiler.CompilationTask compilationTask = compiler.getTask(
null,
fileManager,
DIAGNOSTIC_COLLECTOR,
options,
null,
Collections.singletonList(javaFileObject)
);
Boolean result = compilationTask.call();
System.out.println(String.format("编译[%s]结果:%s", qualifiedName, result));
Class<?> klass = classLoader.loadClass(qualifiedName);
return (T) klass.getDeclaredConstructor().newInstance();
}
}

完成上面工具的搭建之后。我们可以接入数据库的操作了。数据库层面省略,只展示 service 层


service 层:


public class JavaService {

public Object saveAndGetObject(String packageName,String className,String javaContent) throws Exception {
Object object = JdkCompiler.compile(packageName, className, javaContent);
return object;
}

}

测试:


public class TestService {

public static void main(String[] args) throws Exception {
test();
}

static String content="package cn.mmc;\n" +
"\n" +
"public class SayHello {\n" +
" \n" +
" public void say(){\n" +
" System.out.println(\"11111111111\");\n" +
" }\n" +
"}";

static String content2="package cn.mmc;\n" +
"\n" +
"public class SayHello {\n" +
" \n" +
" public void say(){\n" +
" System.out.println(\"22222222222222\");\n" +
" }\n" +
"}";

public static void test() throws Exception {
JavaService javaService = new JavaService();
Object sayHello = javaService.saveAndGetObject("cn.mmc", "SayHello", content);
sayHello.getClass().getMethod("say").invoke(sayHello);

Object sayHello2 = javaService.saveAndGetObject("cn.mmc", "SayHello", content2);
sayHello2.getClass().getMethod("say").invoke(sayHello2);
}
}

我们在启动应用时,更换了代码文件内存,然后直接反射调用对象的方法。执行结果:


可以看到,新的代码已经生效!!!



注意,直接开放修改代码虽然方便,但是一定要做好安全防护


作者:女友在高考
来源:juejin.cn/post/7134155429147312141

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