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