我们有能力动态编译Java代码。我至少知道java-runtime compiler和inmemoryjavacompiler
,但似乎它们无法编译依赖某些类负载的某些类的类。
是否可以动态编译仅在特定classloader中可用的类的Java代码?假设:
ClassLoader classloader = ... // only this CL can load class 'com.External'
String source = "public class MyClass extends com.External {}";
Class<?> compiled = DesiredDynamicCompiler.compile("MyClass", source, classloader);
// last argument is like an information to compiler where to search all dependencies
提供更多的见解:我想在java中做什么,groovyClassloader在grovy中可以做什么:
GroovyClassLoader groovyClassLoader = new GroovyClassLoader(classLoader);
Class<?> parsedClass = groovyClassLoader.parseClass("some source");
该代码可以解析仅取决于仅在指定的classloader中可用的类的类。
无法使用ClassLoader
作为参考,除非它能够提供其定义类的类字节。即,如果您具有代表顶级类的Class
实例,则可以使用classInstance.getResourceAsStream(classInstance.getSimpleName()+".class")
尝试使用类字节。如果您可以访问组成动态类的字节,则可以通过JavaFileManager
实现使Java编译器可用。
编译器API是标准API的一部分,不需要第三方库。以下代码通过首先编译测试类来证明这一点
// customize these, if you want, null triggers default behavior
DiagnosticListener<JavaFileObject> diagnosticListener = null;
Locale locale = null;
// the first class, to be present at runtime only
String class1 = "package test;npublic class Class1 {}";
JavaCompiler c = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager fm
= c.getStandardFileManager(diagnosticListener, locale, Charset.defaultCharset());
// define where to store compiled class files - use a temporary directory
fm.setLocation(StandardLocation.CLASS_OUTPUT, Collections.singleton(
Files.createTempDirectory("compile-test").toFile()));
JavaCompiler.CompilationTask task = c.getTask(null, fm,
diagnosticListener, Collections.emptySet(), Collections.emptySet(),
Collections.singleton(new SimpleJavaFileObject(
URI.create("string:///Class1.java"), Kind.SOURCE) {
public CharSequence getCharContent(boolean ignoreEncodingErrors) {
return class1;
}
}));
if(task.call()) {
FileObject fo = fm.getJavaFileForInput(
StandardLocation.CLASS_OUTPUT, "test.Class1", Kind.CLASS);
// these are the class bytes of the first class
byte[] class1bytes = Files.readAllBytes(Paths.get(fo.toUri()));
// the actual task: define a class dependent on the first class
String class2 = "package test;npublic class Class2 { Class1 variable; }";
// create a file object representing the dynamic class
JavaFileObject jo = new SimpleJavaFileObject(
URI.create("runtime:///test/Class1.class"), Kind.CLASS) {
@Override public InputStream openInputStream() throws IOException {
return new ByteArrayInputStream(class1bytes);
}
};
// and a custom file manager knowing how to locate that class
JavaFileManager myFM = new ForwardingJavaFileManager(fm) {
@Override
public JavaFileObject getJavaFileForInput(
JavaFileManager.Location location, String className, Kind kind)
throws IOException {
if(location==StandardLocation.CLASS_PATH&&className.equals("test.Class1")) {
return jo;
}
return super.getJavaFileForInput(location, className, kind);
}
@Override
public boolean hasLocation(JavaFileManager.Location location) {
return location==StandardLocation.CLASS_PATH || super.hasLocation(location);
}
@Override
public Iterable list(JavaFileManager.Location location,
String packageName, Set kinds, boolean recurse) throws IOException {
if(location==StandardLocation.CLASS_PATH
&& (packageName.equals("test") || recurse&&packageName.isEmpty())) {
return Collections.singleton(jo);
}
return super.list(location, packageName, kinds, recurse);
}
@Override
public String inferBinaryName(
JavaFileManager.Location location, JavaFileObject file) {
if(file==jo) return "test.Class1";
return super.inferBinaryName(location, file);
}
};
// compile the second class using the custom file manager to locate dependencies
task = c.getTask(null, myFM,
diagnosticListener, Collections.emptySet(), Collections.emptySet(),
Collections.singleton(new SimpleJavaFileObject(
URI.create("string:///Class2.java"), Kind.SOURCE) {
public CharSequence getCharContent(boolean ignoreEncodingErrors) {
return class2;
}
}));
if(task.call()) {
fo = fm.getJavaFileForInput(
StandardLocation.CLASS_OUTPUT, "test.Class2", Kind.CLASS);
// there we have the compiled second class
byte[] class2bytes = Files.readAllBytes(Paths.get(fo.toUri()));
}
}
当然,这仅是为了证明原则。您肯定想为文件对象创建出厂方法,并使用Map
s记住它们,等等。
也可以用自定义内存存储替换临时目录。但是关键点仍然是编译器需要能够访问类字节。它不会使用已加载的运行时类。
您应该在类路径上具有所有依赖关系。无论如何,您引用的工具在封面下使用Java编译器API。
它不与当前JVM内存中的类交互,它仅搜索classPath中的依赖项。
您可以关注compilerutils-> com.sun.tools.javac.api.javactool->再以某种感觉。
您可以尝试做的一件事是将您动态编译的依赖项倾倒到ClassPath中的适当位置为.class文件,以便您的汇编过程会拾取它们。