如何构建回测系统（八）

代码升级问题

回测系统必然涉及策略的反复测试及重新部署

重写ClassLoader

为了将代码存放在不同的位置，我们需要重写ClassLoader用以加载二进制代码，我们选用数据库作为代码的存放处，方便查询

重写Classloader中findClass函数

需要注意关键点是，defineClass对同一个class只能解析一次（JVM限制），所以我们需要解析后将Class用Map缓存起来

如果需要更新同名class，需要将Classloader丢弃掉，并新建一个ClassLoader实例（JVM在认为一个相同的Class是指相同的Classloader+全限名则是相同）

实现JavaCompilerInMemory

为了让前端页面能动态的修改class类，需要将.java的内容通过Java代码进行编译
参考代码：

    public byte[] compile() {
        ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        var diagnosticCollector = new DiagnosticCollector<JavaFileObject>();
        var fileManager = new MyJavaFileObjectManager(compiler.getStandardFileManager(diagnosticCollector, null, StandardCharsets.UTF_8));
        var javaFileObjects = Collections.singletonList(new MyJavaFileObject(getName(), codes, byteArrayOutputStream));
        fileManager.outputStream = byteArrayOutputStream;

        var compilerTask = compiler.getTask(null, fileManager, diagnosticCollector, null, null, javaFileObjects);
        if (compilerTask.call()) {
            return byteArrayOutputStream.toByteArray();
        } else {
            StringBuilder stringBuilder = new StringBuilder();
            diagnosticCollector.getDiagnostics().forEach(diagnostic -> stringBuilder.append(diagnostic.getMessage(Locale.CHINESE)));
            throw new RunException(stringBuilder.toString());
        }
    }

    static class MyJavaFileObjectManager extends ForwardingJavaFileManager<JavaFileManager> {

        Map<String, JavaFileObject> fileObjects = new HashMap<>();
        OutputStream outputStream;

        public MyJavaFileObjectManager(JavaFileManager fileManager) {
            super(fileManager);
        }

        @Override
        public JavaFileObject getJavaFileForInput(Location location, String className, JavaFileObject.Kind kind) throws IOException {
            JavaFileObject javaFileObject = fileObjects.get(className);
            if (javaFileObject == null) {
                return super.getJavaFileForInput(location, className, kind);
            }
            return javaFileObject;
        }

        @Override
        public JavaFileObject getJavaFileForOutput(Location location, String qualifiedClassName, JavaFileObject.Kind kind, FileObject sibling) throws IOException {
            JavaFileObject javaFileObject = new MyJavaFileObject(qualifiedClassName, kind, outputStream);
            fileObjects.put(qualifiedClassName, javaFileObject);
            return javaFileObject;
        }
    }

    static class MyJavaFileObject extends SimpleJavaFileObject {
        String source;
        OutputStream outputStream;
        String name;

        public MyJavaFileObject(String name, Kind kind, OutputStream stream) {
            super(URI.create("String:///" + name + kind.extension), kind);
            this.outputStream = stream;
        }

        public MyJavaFileObject(String name, String source) {
            super(URI.create("String:///" + name + Kind.SOURCE.extension), Kind.SOURCE);
            this.source = source;
            this.name = name;
        }

        public MyJavaFileObject(String name, String source, OutputStream outputStream) {
            this(name, source);
            this.outputStream = outputStream;
        }

        public MyJavaFileObject(URI uri, Kind kind) {
            super(uri, kind);
        }

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

        @Override
        public OutputStream openOutputStream() throws IOException {
            return outputStream;
        }
    }

最终

通过CURD将上述的ClassLoader和JavaCompilerInMemory通过自己的业务穿起来，就可以实现在网页编写java代码，并动态修改和运行