Related
I'm loading in classes from a JAR that implement an interface from a public API. The interface itself will remain constant but other classes associated with the API may change over time. Clearly once the API changes we will no longer be able to support implementations of the interface that were written with the old version. However some of the interface methods provide simple meta-data of type String that we can assume will never change and never rely on the other parts of the API that may change. I would like to be able to extract this meta-data even when the API has changed.
For example consider the following implementation that might be loaded in where Foo is the interface and Bar is an another class in the API. I want to call the name method even when the class Bar no longer exists.
class MyFoo implements Foo {
Bar bar = null;
#Override public String name() {
return "MyFoo"
}
}
As far as I can see the obvious approach is to override loadClass(String name) in my custom ClassLoader and return some "fake" class for Bar. The meta-data methods can be assumed to never create or use a Bar object. The question is how to generate this "fake" class when asked to load Bar. I've thought about the following approaches:
Simply return any old existing class. I've tried returning Object.class but this still results in a NoClassDefFoundError for Bar when I try to instantiate an instance of Foo.
Use ASM to generate the byte code for a new class from scratch.
Use ASM to rename some sort of empty template class to match Bar and load that.
Both 2. and 3. seem quite involved, so I was wondering if there was an easier way to achieve my goal?
Here is a class loader which will create a dummy class for every class it didn’t find on the search path, in a very simple way:
public class DummyGeneratorLoader extends URLClassLoader {
public DummyGeneratorLoader(URL[] urls, ClassLoader parent) {
super(urls, parent);
}
public DummyGeneratorLoader(URL[] urls) {
super(urls);
}
public DummyGeneratorLoader(
URL[] urls, ClassLoader parent, URLStreamHandlerFactory factory) {
super(urls, parent, factory);
}
static final byte[] template = ("Êþº¾\0\0\0002\0\n\1\7\0\1\1\0\20java/lang/Object"
+ "\7\0\3\1\0\6<init>\1\0\3()V\14\0\5\0\6\n\0\4\0\7\1\0\4Code\0\1\0\2\0\4\0"
+ "\0\0\0\0\1\0\1\0\5\0\6\0\1\0\t\0\0\0\21\0\1\0\1\0\0\0\5*·\0\b±\0\0\0\0\0\0")
.getBytes(StandardCharsets.ISO_8859_1);
#Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
try {
return super.findClass(name);
}
catch(ClassNotFoundException ex) { }
return new ByteArrayOutputStream(template.length + name.length() + 10) { {
write(template, 0, 11);
try { new DataOutputStream(this).writeUTF(name.replace('.', '/')); }
catch (IOException ex) { throw new AssertionError(); }
write(template, 11, template.length - 11);
}
Class<?> toClass(String name) {
return defineClass(name, buf, 0, count); } }.toClass(name);
}
}
However, there might be a lot of expectations or structural constraints imposed by the using code which the dummy class can’t fulfill. After all, before you can invoke the interface method, you have to create an instance of the class, so it has to pass verification and a successful execution of its constructor.
If the methods truly have the assumed structure like public String name() { return "MyFoo"; } using ASM may be the simpler choice, but not to generate an arbitrarily complex fake environment, but to parse these methods and predict the constant value they’d return. Such a method would consist of two instructions only, ldc value and areturn. You only need to check that this is the case and extract the value from the first instruction.
How does one go about and try to find all subclasses of a given class (or all implementors of a given interface) in Java?
As of now, I have a method to do this, but I find it quite inefficient (to say the least).
The method is:
Get a list of all class names that exist on the class path
Load each class and test to see if it is a subclass or implementor of the desired class or interface
In Eclipse, there is a nice feature called the Type Hierarchy that manages to show this quite efficiently.
How does one go about and do it programmatically?
Scanning for classes is not easy with pure Java.
The spring framework offers a class called ClassPathScanningCandidateComponentProvider that can do what you need. The following example would find all subclasses of MyClass in the package org.example.package
ClassPathScanningCandidateComponentProvider provider = new ClassPathScanningCandidateComponentProvider(false);
provider.addIncludeFilter(new AssignableTypeFilter(MyClass.class));
// scan in org.example.package
Set<BeanDefinition> components = provider.findCandidateComponents("org/example/package");
for (BeanDefinition component : components)
{
Class cls = Class.forName(component.getBeanClassName());
// use class cls found
}
This method has the additional benefit of using a bytecode analyzer to find the candidates which means it will not load all classes it scans.
There is no other way to do it other than what you described. Think about it - how can anyone know what classes extend ClassX without scanning each class on the classpath?
Eclipse can only tell you about the super and subclasses in what seems to be an "efficient" amount of time because it already has all of the type data loaded at the point where you press the "Display in Type Hierarchy" button (since it is constantly compiling your classes, knows about everything on the classpath, etc).
This is not possible to do using only the built-in Java Reflections API.
A project exists that does the necessary scanning and indexing of your classpath so you can get access this information...
Reflections
A Java runtime metadata analysis, in the spirit of Scannotations
Reflections scans your classpath, indexes the metadata, allows you to query it on runtime and may save and collect that information for many modules within your project.
Using Reflections you can query your metadata for:
get all subtypes of some type
get all types annotated with some annotation
get all types annotated with some annotation, including annotation parameters matching
get all methods annotated with some
(disclaimer: I have not used it, but the project's description seems to be an exact fit for your needs.)
Try ClassGraph. (Disclaimer, I am the author). ClassGraph supports scanning for subclasses of a given class, either at runtime or at build time, but also much more. ClassGraph can build an abstract representation of the entire class graph (all classes, annotations, methods, method parameters, and fields) in memory, for all classes on the classpath, or for classes in selected packages, and you can query this class graph however you want. ClassGraph supports more classpath specification mechanisms and classloaders than any other scanner, and also works seamlessly with the new JPMS module system, so if you base your code on ClassGraph, your code will be maximally portable. See the API here.
Don't forget that the generated Javadoc for a class will include a list of known subclasses (and for interfaces, known implementing classes).
I know I'm a few years late to this party, but I came across this question trying to solve the same problem. You can use Eclipse's internal searching programatically, if you're writing an Eclipse Plugin (and thus take advantage of their caching, etc), to find classes which implement an interface. Here's my (very rough) first cut:
protected void listImplementingClasses( String iface ) throws CoreException
{
final IJavaProject project = <get your project here>;
try
{
final IType ifaceType = project.findType( iface );
final SearchPattern ifacePattern = SearchPattern.createPattern( ifaceType, IJavaSearchConstants.IMPLEMENTORS );
final IJavaSearchScope scope = SearchEngine.createWorkspaceScope();
final SearchEngine searchEngine = new SearchEngine();
final LinkedList<SearchMatch> results = new LinkedList<SearchMatch>();
searchEngine.search( ifacePattern,
new SearchParticipant[]{ SearchEngine.getDefaultSearchParticipant() }, scope, new SearchRequestor() {
#Override
public void acceptSearchMatch( SearchMatch match ) throws CoreException
{
results.add( match );
}
}, new IProgressMonitor() {
#Override
public void beginTask( String name, int totalWork )
{
}
#Override
public void done()
{
System.out.println( results );
}
#Override
public void internalWorked( double work )
{
}
#Override
public boolean isCanceled()
{
return false;
}
#Override
public void setCanceled( boolean value )
{
}
#Override
public void setTaskName( String name )
{
}
#Override
public void subTask( String name )
{
}
#Override
public void worked( int work )
{
}
});
} catch( JavaModelException e )
{
e.printStackTrace();
}
}
The first problem I see so far is that I'm only catching classes which directly implement the interface, not all their subclasses - but a little recursion never hurt anyone.
I did this several years ago. The most reliable way to do this (i.e. with official Java APIs and no external dependencies) is to write a custom doclet to produce a list that can be read at runtime.
You can run it from the command line like this:
javadoc -d build -doclet com.example.ObjectListDoclet -sourcepath java/src -subpackages com.example
or run it from ant like this:
<javadoc sourcepath="${src}" packagenames="*" >
<doclet name="com.example.ObjectListDoclet" path="${build}"/>
</javadoc>
Here's the basic code:
public final class ObjectListDoclet {
public static final String TOP_CLASS_NAME = "com.example.MyClass";
/** Doclet entry point. */
public static boolean start(RootDoc root) throws Exception {
try {
ClassDoc topClassDoc = root.classNamed(TOP_CLASS_NAME);
for (ClassDoc classDoc : root.classes()) {
if (classDoc.subclassOf(topClassDoc)) {
System.out.println(classDoc);
}
}
return true;
}
catch (Exception ex) {
ex.printStackTrace();
return false;
}
}
}
For simplicity, I've removed command line argument parsing and I'm writing to System.out rather than a file.
Keeping in mind the limitations mentioned in the other answers, you can also use openpojo's PojoClassFactory (available on Maven) in the following manner:
for(PojoClass pojoClass : PojoClassFactory.enumerateClassesByExtendingType(packageRoot, Superclass.class, null)) {
System.out.println(pojoClass.getClazz());
}
Where packageRoot is the root String of the packages you wish to search in (e.g. "com.mycompany" or even just "com"), and Superclass is your supertype (this works on interfaces as well).
Depending on your particular requirements, in some cases Java's service loader mechanism might achieve what you're after.
In short, it allows developers to explicitly declare that a class subclasses some other class (or implements some interface) by listing it in a file in the JAR/WAR file's META-INF/services directory. It can then be discovered using the java.util.ServiceLoader class which, when given a Class object, will generate instances of all the declared subclasses of that class (or, if the Class represents an interface, all the classes implementing that interface).
The main advantage of this approach is that there is no need to manually scan the entire classpath for subclasses - all the discovery logic is contained within the ServiceLoader class, and it only loads the classes explicitly declared in the META-INF/services directory (not every class on the classpath).
There are, however, some disadvantages:
It won't find all subclasses, only those that are explicitly declared. As such, if you need to truly find all subclasses, this approach may be insufficient.
It requires the developer to explicitly declare the class under the META-INF/services directory. This is an additional burden on the developer, and can be error-prone.
The ServiceLoader.iterator() generates subclass instances, not their Class objects. This causes two issues:
You don't get any say on how the subclasses are constructed - the no-arg constructor is used to create the instances.
As such, the subclasses must have a default constructor, or must explicity declare a no-arg constructor.
Apparently Java 9 will be addressing some of these shortcomings (in particular, the ones regarding instantiation of subclasses).
An Example
Suppose you're interested in finding classes that implement an interface com.example.Example:
package com.example;
public interface Example {
public String getStr();
}
The class com.example.ExampleImpl implements that interface:
package com.example;
public class ExampleImpl implements Example {
public String getStr() {
return "ExampleImpl's string.";
}
}
You would declare the class ExampleImpl is an implementation of Example by creating a file META-INF/services/com.example.Example containing the text com.example.ExampleImpl.
Then, you could obtain an instance of each implementation of Example (including an instance of ExampleImpl) as follows:
ServiceLoader<Example> loader = ServiceLoader.load(Example.class)
for (Example example : loader) {
System.out.println(example.getStr());
}
// Prints "ExampleImpl's string.", plus whatever is returned
// by other declared implementations of com.example.Example.
It should be noted as well that this will of course only find all those subclasses that exist on your current classpath. Presumably this is OK for what you are currently looking at, and chances are you did consider this, but if you have at any point released a non-final class into the wild (for varying levels of "wild") then it is entirely feasible that someone else has written their own subclass that you will not know about.
Thus if you happened to be wanting to see all subclasses because you want to make a change and are going to see how it affects subclasses' behaviour - then bear in mind the subclasses that you can't see. Ideally all of your non-private methods, and the class itself should be well-documented; make changes according to this documentation without changing the semantics of methods/non-private fields and your changes should be backwards-compatible, for any subclass that followed your definition of the superclass at least.
The reason you see a difference between your implementation and Eclipse is because you scan each time, while Eclipse (and other tools) scan only once (during project load most of the times) and create an index. Next time you ask for the data it doesn't scan again, but look at the index.
I'm using a reflection lib, which scans your classpath for all subclasses: https://github.com/ronmamo/reflections
This is how it would be done:
Reflections reflections = new Reflections("my.project");
Set<Class<? extends SomeType>> subTypes = reflections.getSubTypesOf(SomeType.class);
You can use org.reflections library and then, create an object of Reflections class. Using this object, you can get list of all subclasses of given class.
https://www.javadoc.io/doc/org.reflections/reflections/0.9.10/org/reflections/Reflections.html
Reflections reflections = new Reflections("my.project.prefix");
System.out.println(reflections.getSubTypesOf(A.class)));
Add them to a static map inside (this.getClass().getName()) the parent classes constructor (or create a default one) but this will get updated in runtime. If lazy initialization is an option you can try this approach.
I just write a simple demo to use the org.reflections.Reflections to get subclasses of abstract class:
https://github.com/xmeng1/ReflectionsDemo
I needed to do this as a test case, to see if new classes had been added to the code. This is what I did
final static File rootFolder = new File(SuperClass.class.getProtectionDomain().getCodeSource().getLocation().getPath());
private static ArrayList<String> files = new ArrayList<String>();
listFilesForFolder(rootFolder);
#Test(timeout = 1000)
public void testNumberOfSubclasses(){
ArrayList<String> listSubclasses = new ArrayList<>(files);
listSubclasses.removeIf(s -> !s.contains("Superclass.class"));
for(String subclass : listSubclasses){
System.out.println(subclass);
}
assertTrue("You did not create a new subclass!", listSubclasses.size() >1);
}
public static void listFilesForFolder(final File folder) {
for (final File fileEntry : folder.listFiles()) {
if (fileEntry.isDirectory()) {
listFilesForFolder(fileEntry);
} else {
files.add(fileEntry.getName().toString());
}
}
}
If you intend to load all subclassess of given class which are in the same package, you can do so:
public static List<Class> loadAllSubClasses(Class pClazz) throws IOException, ClassNotFoundException {
ClassLoader classLoader = pClazz.getClassLoader();
assert classLoader != null;
String packageName = pClazz.getPackage().getName();
String dirPath = packageName.replace(".", "/");
Enumeration<URL> srcList = classLoader.getResources(dirPath);
List<Class> subClassList = new ArrayList<>();
while (srcList.hasMoreElements()) {
File dirFile = new File(srcList.nextElement().getFile());
File[] files = dirFile.listFiles();
if (files != null) {
for (File file : files) {
String subClassName = packageName + '.' + file.getName().substring(0, file.getName().length() - 6);
if (! subClassName.equals(pClazz.getName())) {
subClassList.add(Class.forName(subClassName));
}
}
}
}
return subClassList;
}
find all classes in classpath
public static List<String> getClasses() {
URLClassLoader urlClassLoader = (URLClassLoader) Thread.currentThread().getContextClassLoader();
List<String> classes = new ArrayList<>();
for (URL url : urlClassLoader.getURLs()) {
try {
if (url.toURI().getScheme().equals("file")) {
File file = new File(url.toURI());
if (file.exists()) {
try {
if (file.isDirectory()) {
for (File listFile : FileUtils.listFiles(file, new String[]{"class"}, true)) {
String classFile = listFile.getAbsolutePath().replace(file.getAbsolutePath(), "").replace(".class", "");
if (classFile.startsWith(File.separator)) {
classFile = classFile.substring(1);
}
classes.add(classFile.replace(File.separator, "."));
}
} else {
JarFile jarFile = new JarFile(file);
if (url.getFile().endsWith(".jar")) {
Enumeration<JarEntry> entries = jarFile.entries();
while (entries.hasMoreElements()) {
JarEntry jarEntry = entries.nextElement();
if (jarEntry.getName().endsWith(".class")) {
classes.add(jarEntry.getName().replace(".class", "").replace("/", "."));
}
}
}
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
} catch (URISyntaxException e) {
e.printStackTrace();
}
}
return classes;
}
enter link description hereService Manager in java will get all implementing classes for an interface in J
I'am trying to make a OGM to translate object to Vertex for the OrientDB. Currently i'am using GCLib but i read that ByteBuddy could implements two critical things that if work, it will improve the OGM speed.
Could ByteBuddy implement field access control? I read the doc but it's not clear or I do not understand it.
Dinamically add default empty constructor.
The current problem is this: We do not know the class definition that will be passed as a parameter. The idea is to redefine the class and implement the empty constructor if it not have one, add a field named __BB__Dirty to set the object as dirty if an assign operation was detected and force the implementation of an interface to talk with the object.
Example:
A generic class:
public class Example {
int i = 0;
String stringField;
public Example(Strinf s) {
stringField = s;
}
public void addToI(){
i++;
}
}
Now we have an interface like this:
public interface DirtyCheck {
public boolean isDirty();
}
So, I want to force the Example class to implement the interface, the method isDirty(), a field to work on and a default contructor so the class should be translated to:
public class Example implements DirtyCheck {
int i = 0;
String stringField;
boolean __BB__dirty = false;
public Example() {
}
public Example(Strinf s) {
stringField = s;
}
public void addToI(){
i++;
}
public boolean isDirty() {
return this.__BB__dirty;
}
}
and the some magically assigner so if any field (except __BB__dirty) is modified, the __BB__dirty field is set to True;
I have tried the first part of this but I fail :(
...
ByteBuddyAgent.install();
Example ex = new ByteBuddy()
.redefine(Example.class)
.defineField("__BB__Dirty", boolean.class, Visibility.PUBLIC)
.make()
.load(Example.class.getClassLoader(), ClassReloadingStrategy.fromInstalledAgent())
.getLoaded().newInstance();
....
ex.addToI(); // <--- this should set __BB__dirty to true since it
// assign a value to i.
But i get this error:
Exception in thread "main" java.lang.UnsupportedOperationException: class redefinition failed: attempted to change the schema (add/remove fields)
at sun.instrument.InstrumentationImpl.redefineClasses0(Native Method)
at sun.instrument.InstrumentationImpl.redefineClasses(InstrumentationImpl.java:170)
at net.bytebuddy.dynamic.loading.ClassReloadingStrategy$Strategy$1.apply(ClassReloadingStrategy.java:297)
at net.bytebuddy.dynamic.loading.ClassReloadingStrategy.load(ClassReloadingStrategy.java:173)
at net.bytebuddy.dynamic.DynamicType$Default$Unloaded.load(DynamicType.java:4350)
at Test.TestBB.<init>(TestBB.java:33)
at Test.TestBB.main(TestBB.java:23)
I'am stuck in the very first stage to solve the problem with BB.
Thanks
The Java virtual machine does not support changing the layout of classes that are already loaded when redefining a class. This is not a limitation of Byte Buddy but the VM implementation.
In order to do what you want, you should look at the AgentBuilder API which allows you to modify classes before they are loaded. Creating an agent does however require you to add it explicitly as an agent on startup (opposed to adding the library to the class path.
You can implement the interface by calling:
.implement(DirtyCheck.class).intercept(FieldAccessor.of("__dirty__");
You can also add a default constructor by simply defining one:
.defineConstructor(Visibility.PUBLIC).intercept(SuperMethodCall.INSTANCE)
The latter definition requires the super class to define a default constructor.
Introduction
As a disclaimer, I'v read Why can't static methods be abstract in Java and, even if I respectfully disagree with the accepted answer about a "logical contradiction", I don't want any answer about the usefulness of static abstract just an answer to my question ;)
I have a class hierarchy representing some tables from a database. Each class inherits the Entity class which contains a lot of utility methods for accessing the database, creating queries, escaping characters, etc.
Each instance of a class is a row from the database.
The problem
Now, in order to factorize as much code as possible, I want to add information about related columns and table name for each class. These informations must be accessible without a class instance and will be used in Entity to build queries among other things.
The obvious way to store these data are static fields returned by static methods in each class. Problem is you can't force the class to implement these static methods and you can't do dynamic linking on static methods call in Java.
My Solutions
Use a HashMap, or any similar data structure, to hold the informations. Problem : if informations are missing error will be at runtime not compile time.
Use a parallel class hierarchy for the utility function where each corresponding class can be instantiated and dynamic linking used. Problem : code heavy, runtime error if the class don't exist
The question
How will you cope with the absence of abstract static and dynamic linking on abstract method ?
In a perfect world, the given solution should generate a compile error if the informations for a class are missing and data should be easily accessible from withing the Entity class.
The answer doesn't need to be in Java, C# is also ok and any insight on how to do this without some specific code in any language will be welcomed.
Just to be clear, I don't have any requirement at all besides simplicity. Nothing have to be static. I only want to retrieve table and columns name from Entity to build a query.
Some code
class Entity {
public static function afunction(Class clazz) { // this parameter is an option
// here I need to have access to table name of any children of Entity
}
}
class A extends Entity {
static String table = "a";
}
class B extends Entity {
static String table = "b";
}
You should use the Java annotation coupled with the javac annotation processor, as it's the most efficient solution. It's however a bit more complicated than the usual annotation paradigm.
This link shows you how you can implement an annotation processor that will be used at the compile time.
If I reuse your example, I'd go this way:
#Target(ElementType.TYPE)
#Retention(RetentionType.SOURCE)
#interface MetaData {
String table();
}
abstract class Entity {}
#MetaData(table="a")
class A extends Entity {}
#MetaData(table="b")
class B extends Entity {}
class EntityGetter {
public <E extends Entity> E getEntity(Class<E> type) {
MetaData metaData = type.getAnnotation(MetaData.class);
if (metaData == null) {
throw new Error("Should have been compiled with the preprocessor.");
// Yes, do throw an Error. It's a compile-time error, not a simple exceptional condition.
}
String table = metaData.table();
// do whatever you need.
}
}
In your annotation processing, you then should check whether the annotation is set, whether the values are correct, and make the compilation fail.
The complete documentation is available in the documentation for the package javax.annotation.processing.
Also, a few tutorials are available on the Internet if you search for "java annotation processing".
I will not go deeper in the subject as I never used the technology myself before.
I have run into the same problems as you, and am using the following approach now. Store Metadata about columns as annotations and parse them at runtime. Store this information in a map. If you really want compile time errors to appear, most IDEs (Eclipse e.g.) support custom builder types, that can validate the classes during build time.
You could also use the compile time annotation processing tool which comes with java, which can also be integrated into the IDE builds. Read into it and give it a try.
In Java the most similar approach to "static classes" are the static enums.
The enum elements are handed as static constants, so they can be accesed from any static context.
The enum can define one or more private constructors, accepting some intialization parameters (as it could be a table name, a set of columns, etc).
The enum class can define abstract methods, which must be implemented by the concrete elements, in order to compile.
public enum EntityMetadata {
TABLE_A("TableA", new String[]{"ID", "DESC"}) {
#Override
public void doSomethingWeirdAndExclusive() {
Logger.getLogger(getTableName()).info("I'm positively TableA Metadata");
}
},
TABLE_B("TableB", new String[]{"ID", "AMOUNT", "CURRENCY"}) {
#Override
public void doSomethingWeirdAndExclusive() {
Logger.getLogger(getTableName()).info("FOO BAR message, or whatever");
}
};
private String tableName;
private String[] columnNames;
private EntityMetadata(String aTableName, String[] someColumnNames) {
tableName=aTableName;
columnNames=someColumnNames;
}
public String getTableName() {
return tableName;
}
public String[] getColumnNames() {
return columnNames;
}
public abstract void doSomethingWeirdAndExclusive();
}
Then to access a concrete entity metadata this would be enough:
EntityMetadata.TABLE_B.doSomethingWeirdAndExclusive();
You could also reference them from an Entity implemetation, forcing each to refer an EntityMetadata element:
abstract class Entity {
public abstract EntityMetadata getMetadata();
}
class A extends Entity {
public EntityMetadata getMetadata() {
return EntityMetadata.TABLE_A;
}
}
class B extends Entity {
public EntityMetadata getMetadata() {
return EntityMetadata.TABLE_B;
}
}
IMO, this approach will be fast and light-weight.
The dark side of it is that if your enum type needs to be really complex, with lot of different params, or a few different complex overriden methods, the source code for the enum can become a little messy.
Mi idea, is to skip the tables stuff, and relate to the "There are not abstract static methods". Use "pseudo-abstract-static" methods.
First define an exception that will ocurr when an abstract static method is executed:
public class StaticAbstractCallException extends Exception {
StaticAbstractCallException (String strMessage){
super(strMessage);
}
public String toString(){
return "StaticAbstractCallException";
}
} // class
An "abstract" method means it will be overriden in subclasses, so you may want to define a base class, with static methods that are suppouse to be "abstract".
abstract class MyDynamicDevice {
public static void start() {
throw new StaticAbstractCallException("MyDynamicDevice.start()");
}
public static void doSomething() {
throw new StaticAbstractCallException("MyDynamicDevice.doSomething()");
}
public static void finish() {
throw new StaticAbstractCallException("MyDynamicDevice.finish()");
}
// other "abstract" static methods
} // class
...
And finally, define the subclasses that override the "pseudo-abstract" methods.
class myPrinterBrandDevice extends MyDynamicDevice {
public static void start() {
// override MyStaticLibrary.start()
}
/*
// ops, we forgot to override this method !!!
public static void doSomething() {
// ...
}
*/
public static void finish() {
// override MyStaticLibrary.finish()
}
// other abstract static methods
} // class
When the static myStringLibrary doSomething is called, an exception will be generated.
I do know of a solution providing all you want, but it's a huge hack I wouldn't want in my own code nowadays:
If Entity may be abstract, simply add your methods providing the meta data to that base class and declare them abstract.
Otherwise create an interface, with methods providing all your data like this
public interface EntityMetaData{
public String getTableName();
...
}
All subclasses of Entity would have to implement this interface though.
Now your problem is to call these methods from your static utility method, since you don't have an instance there. So you need to create an instance. Using Class.newInstance() is not feasable, since you'd need a nullary constructor, and there might be expensive initialization or initialization with side-effects happening in the constructor, you don't want to trigger.
The hack I propose is to use Objenesis to instantiate your Class. This library allows instatiating any class, without calling the constructor. There's no need for a nullary constructor either. They do this with some huge hacks internally, which are adapted for all major JVMs.
So your code would look like this:
public static function afunction(Class clazz) {
Objenesis objenesis = new ObjenesisStd();
ObjectInstantiator instantiator = objenesis.getInstantiatorOf(clazz);
Entity entity = (Entity)instantiator.newInstance();
// use it
String tableName = entity.getTableName();
...
}
Obviously you should cache your instances using a Map<Class,Entity>, which reduces the runtime cost to practically nothing (a single lookup in your caching map).
I am using Objenesis in one project of my own, where it enabled me to create a beautiful, fluent API. That was such a big win for me, that I put up with this hack. So I can tell you, that it really works. I used my library in many environments with many different JVM versions.
But this is not good design! I advise against using such a hack, even if it works for now, it might stop in the next JVM. And then you'll have to pray for an update of Objenesis...
If I were you, I'd rethink my design leading to the whole requirement. Or give up compile time checking and use annotations.
Your requirement to have static method doesn't leave much space for clean solution. One of the possible ways is to mix static and dynamic, and lose some CPU for a price of saving on RAM:
class Entity {
private static final ConcurrentMap<Class, EntityMetadata> metadataMap = new ...;
Entity(EntityMetadata entityMetadata) {
metadataMap.putIfAbsent(getClass(), entityMetadata);
}
public static EntityMetadata getMetadata(Class clazz) {
return metadataMap.get(clazz);
}
}
The way I would like more would be to waste a reference but have it dynamic:
class Entity {
protected final EntityMetadata entityMetadata;
public Entity(EntityMetadata entityMetadata) {
this.entityMetadata=entityMetadata;
}
}
class A extends Entity {
static {
MetadataFactory.setMetadataFor(A.class, ...);
}
public A() {
super(MetadataFactory.getMetadataFor(A.class));
}
}
class MetadataFactory {
public static EntityMetadata getMetadataFor(Class clazz) {
return ...;
}
public static void setMetadataFor(Class clazz, EntityMetadata metadata) {
...;
}
}
You could get even get rid of EntityMetadata in Entity completely and leave it factory only. Yes, it would not force to provide it for each class in compile-time, but you can easily enforce that in the runtime. Compile-time errors are great but they aren't holy cows after all as you'd always get an error immediately if a class hasn't provided a relevant metadata part.
I would have abstracted away all meta data for the entities (table names, column names) to a service not known by the entities them selfs. Would be much cleaner than having that information inside the entities
MetaData md = metadataProvider.GetMetaData<T>();
String tableName = md.getTableName();
First, let me tell you I agree with you I would like to have a way to enforce static method to be present in classes.
As a solution you can "extend" compile time by using a custom ANT task that checks for the presence of such methods, and get error in compilation time. Of course it won't help you inside you IDE, but you can use a customizable static code analyzer like PMD and create a custom rule to check for the same thing.
And there you java compile (well, almost compile) and edit time error checking.
The dynamic linking emulation...well, this is harder. I'm not sure I understand what you mean. Can you write an example of what you expect to happen?
How does one go about and try to find all subclasses of a given class (or all implementors of a given interface) in Java?
As of now, I have a method to do this, but I find it quite inefficient (to say the least).
The method is:
Get a list of all class names that exist on the class path
Load each class and test to see if it is a subclass or implementor of the desired class or interface
In Eclipse, there is a nice feature called the Type Hierarchy that manages to show this quite efficiently.
How does one go about and do it programmatically?
Scanning for classes is not easy with pure Java.
The spring framework offers a class called ClassPathScanningCandidateComponentProvider that can do what you need. The following example would find all subclasses of MyClass in the package org.example.package
ClassPathScanningCandidateComponentProvider provider = new ClassPathScanningCandidateComponentProvider(false);
provider.addIncludeFilter(new AssignableTypeFilter(MyClass.class));
// scan in org.example.package
Set<BeanDefinition> components = provider.findCandidateComponents("org/example/package");
for (BeanDefinition component : components)
{
Class cls = Class.forName(component.getBeanClassName());
// use class cls found
}
This method has the additional benefit of using a bytecode analyzer to find the candidates which means it will not load all classes it scans.
There is no other way to do it other than what you described. Think about it - how can anyone know what classes extend ClassX without scanning each class on the classpath?
Eclipse can only tell you about the super and subclasses in what seems to be an "efficient" amount of time because it already has all of the type data loaded at the point where you press the "Display in Type Hierarchy" button (since it is constantly compiling your classes, knows about everything on the classpath, etc).
This is not possible to do using only the built-in Java Reflections API.
A project exists that does the necessary scanning and indexing of your classpath so you can get access this information...
Reflections
A Java runtime metadata analysis, in the spirit of Scannotations
Reflections scans your classpath, indexes the metadata, allows you to query it on runtime and may save and collect that information for many modules within your project.
Using Reflections you can query your metadata for:
get all subtypes of some type
get all types annotated with some annotation
get all types annotated with some annotation, including annotation parameters matching
get all methods annotated with some
(disclaimer: I have not used it, but the project's description seems to be an exact fit for your needs.)
Try ClassGraph. (Disclaimer, I am the author). ClassGraph supports scanning for subclasses of a given class, either at runtime or at build time, but also much more. ClassGraph can build an abstract representation of the entire class graph (all classes, annotations, methods, method parameters, and fields) in memory, for all classes on the classpath, or for classes in selected packages, and you can query this class graph however you want. ClassGraph supports more classpath specification mechanisms and classloaders than any other scanner, and also works seamlessly with the new JPMS module system, so if you base your code on ClassGraph, your code will be maximally portable. See the API here.
Don't forget that the generated Javadoc for a class will include a list of known subclasses (and for interfaces, known implementing classes).
I know I'm a few years late to this party, but I came across this question trying to solve the same problem. You can use Eclipse's internal searching programatically, if you're writing an Eclipse Plugin (and thus take advantage of their caching, etc), to find classes which implement an interface. Here's my (very rough) first cut:
protected void listImplementingClasses( String iface ) throws CoreException
{
final IJavaProject project = <get your project here>;
try
{
final IType ifaceType = project.findType( iface );
final SearchPattern ifacePattern = SearchPattern.createPattern( ifaceType, IJavaSearchConstants.IMPLEMENTORS );
final IJavaSearchScope scope = SearchEngine.createWorkspaceScope();
final SearchEngine searchEngine = new SearchEngine();
final LinkedList<SearchMatch> results = new LinkedList<SearchMatch>();
searchEngine.search( ifacePattern,
new SearchParticipant[]{ SearchEngine.getDefaultSearchParticipant() }, scope, new SearchRequestor() {
#Override
public void acceptSearchMatch( SearchMatch match ) throws CoreException
{
results.add( match );
}
}, new IProgressMonitor() {
#Override
public void beginTask( String name, int totalWork )
{
}
#Override
public void done()
{
System.out.println( results );
}
#Override
public void internalWorked( double work )
{
}
#Override
public boolean isCanceled()
{
return false;
}
#Override
public void setCanceled( boolean value )
{
}
#Override
public void setTaskName( String name )
{
}
#Override
public void subTask( String name )
{
}
#Override
public void worked( int work )
{
}
});
} catch( JavaModelException e )
{
e.printStackTrace();
}
}
The first problem I see so far is that I'm only catching classes which directly implement the interface, not all their subclasses - but a little recursion never hurt anyone.
I did this several years ago. The most reliable way to do this (i.e. with official Java APIs and no external dependencies) is to write a custom doclet to produce a list that can be read at runtime.
You can run it from the command line like this:
javadoc -d build -doclet com.example.ObjectListDoclet -sourcepath java/src -subpackages com.example
or run it from ant like this:
<javadoc sourcepath="${src}" packagenames="*" >
<doclet name="com.example.ObjectListDoclet" path="${build}"/>
</javadoc>
Here's the basic code:
public final class ObjectListDoclet {
public static final String TOP_CLASS_NAME = "com.example.MyClass";
/** Doclet entry point. */
public static boolean start(RootDoc root) throws Exception {
try {
ClassDoc topClassDoc = root.classNamed(TOP_CLASS_NAME);
for (ClassDoc classDoc : root.classes()) {
if (classDoc.subclassOf(topClassDoc)) {
System.out.println(classDoc);
}
}
return true;
}
catch (Exception ex) {
ex.printStackTrace();
return false;
}
}
}
For simplicity, I've removed command line argument parsing and I'm writing to System.out rather than a file.
Keeping in mind the limitations mentioned in the other answers, you can also use openpojo's PojoClassFactory (available on Maven) in the following manner:
for(PojoClass pojoClass : PojoClassFactory.enumerateClassesByExtendingType(packageRoot, Superclass.class, null)) {
System.out.println(pojoClass.getClazz());
}
Where packageRoot is the root String of the packages you wish to search in (e.g. "com.mycompany" or even just "com"), and Superclass is your supertype (this works on interfaces as well).
Depending on your particular requirements, in some cases Java's service loader mechanism might achieve what you're after.
In short, it allows developers to explicitly declare that a class subclasses some other class (or implements some interface) by listing it in a file in the JAR/WAR file's META-INF/services directory. It can then be discovered using the java.util.ServiceLoader class which, when given a Class object, will generate instances of all the declared subclasses of that class (or, if the Class represents an interface, all the classes implementing that interface).
The main advantage of this approach is that there is no need to manually scan the entire classpath for subclasses - all the discovery logic is contained within the ServiceLoader class, and it only loads the classes explicitly declared in the META-INF/services directory (not every class on the classpath).
There are, however, some disadvantages:
It won't find all subclasses, only those that are explicitly declared. As such, if you need to truly find all subclasses, this approach may be insufficient.
It requires the developer to explicitly declare the class under the META-INF/services directory. This is an additional burden on the developer, and can be error-prone.
The ServiceLoader.iterator() generates subclass instances, not their Class objects. This causes two issues:
You don't get any say on how the subclasses are constructed - the no-arg constructor is used to create the instances.
As such, the subclasses must have a default constructor, or must explicity declare a no-arg constructor.
Apparently Java 9 will be addressing some of these shortcomings (in particular, the ones regarding instantiation of subclasses).
An Example
Suppose you're interested in finding classes that implement an interface com.example.Example:
package com.example;
public interface Example {
public String getStr();
}
The class com.example.ExampleImpl implements that interface:
package com.example;
public class ExampleImpl implements Example {
public String getStr() {
return "ExampleImpl's string.";
}
}
You would declare the class ExampleImpl is an implementation of Example by creating a file META-INF/services/com.example.Example containing the text com.example.ExampleImpl.
Then, you could obtain an instance of each implementation of Example (including an instance of ExampleImpl) as follows:
ServiceLoader<Example> loader = ServiceLoader.load(Example.class)
for (Example example : loader) {
System.out.println(example.getStr());
}
// Prints "ExampleImpl's string.", plus whatever is returned
// by other declared implementations of com.example.Example.
It should be noted as well that this will of course only find all those subclasses that exist on your current classpath. Presumably this is OK for what you are currently looking at, and chances are you did consider this, but if you have at any point released a non-final class into the wild (for varying levels of "wild") then it is entirely feasible that someone else has written their own subclass that you will not know about.
Thus if you happened to be wanting to see all subclasses because you want to make a change and are going to see how it affects subclasses' behaviour - then bear in mind the subclasses that you can't see. Ideally all of your non-private methods, and the class itself should be well-documented; make changes according to this documentation without changing the semantics of methods/non-private fields and your changes should be backwards-compatible, for any subclass that followed your definition of the superclass at least.
The reason you see a difference between your implementation and Eclipse is because you scan each time, while Eclipse (and other tools) scan only once (during project load most of the times) and create an index. Next time you ask for the data it doesn't scan again, but look at the index.
I'm using a reflection lib, which scans your classpath for all subclasses: https://github.com/ronmamo/reflections
This is how it would be done:
Reflections reflections = new Reflections("my.project");
Set<Class<? extends SomeType>> subTypes = reflections.getSubTypesOf(SomeType.class);
You can use org.reflections library and then, create an object of Reflections class. Using this object, you can get list of all subclasses of given class.
https://www.javadoc.io/doc/org.reflections/reflections/0.9.10/org/reflections/Reflections.html
Reflections reflections = new Reflections("my.project.prefix");
System.out.println(reflections.getSubTypesOf(A.class)));
Add them to a static map inside (this.getClass().getName()) the parent classes constructor (or create a default one) but this will get updated in runtime. If lazy initialization is an option you can try this approach.
I just write a simple demo to use the org.reflections.Reflections to get subclasses of abstract class:
https://github.com/xmeng1/ReflectionsDemo
I needed to do this as a test case, to see if new classes had been added to the code. This is what I did
final static File rootFolder = new File(SuperClass.class.getProtectionDomain().getCodeSource().getLocation().getPath());
private static ArrayList<String> files = new ArrayList<String>();
listFilesForFolder(rootFolder);
#Test(timeout = 1000)
public void testNumberOfSubclasses(){
ArrayList<String> listSubclasses = new ArrayList<>(files);
listSubclasses.removeIf(s -> !s.contains("Superclass.class"));
for(String subclass : listSubclasses){
System.out.println(subclass);
}
assertTrue("You did not create a new subclass!", listSubclasses.size() >1);
}
public static void listFilesForFolder(final File folder) {
for (final File fileEntry : folder.listFiles()) {
if (fileEntry.isDirectory()) {
listFilesForFolder(fileEntry);
} else {
files.add(fileEntry.getName().toString());
}
}
}
If you intend to load all subclassess of given class which are in the same package, you can do so:
public static List<Class> loadAllSubClasses(Class pClazz) throws IOException, ClassNotFoundException {
ClassLoader classLoader = pClazz.getClassLoader();
assert classLoader != null;
String packageName = pClazz.getPackage().getName();
String dirPath = packageName.replace(".", "/");
Enumeration<URL> srcList = classLoader.getResources(dirPath);
List<Class> subClassList = new ArrayList<>();
while (srcList.hasMoreElements()) {
File dirFile = new File(srcList.nextElement().getFile());
File[] files = dirFile.listFiles();
if (files != null) {
for (File file : files) {
String subClassName = packageName + '.' + file.getName().substring(0, file.getName().length() - 6);
if (! subClassName.equals(pClazz.getName())) {
subClassList.add(Class.forName(subClassName));
}
}
}
}
return subClassList;
}
find all classes in classpath
public static List<String> getClasses() {
URLClassLoader urlClassLoader = (URLClassLoader) Thread.currentThread().getContextClassLoader();
List<String> classes = new ArrayList<>();
for (URL url : urlClassLoader.getURLs()) {
try {
if (url.toURI().getScheme().equals("file")) {
File file = new File(url.toURI());
if (file.exists()) {
try {
if (file.isDirectory()) {
for (File listFile : FileUtils.listFiles(file, new String[]{"class"}, true)) {
String classFile = listFile.getAbsolutePath().replace(file.getAbsolutePath(), "").replace(".class", "");
if (classFile.startsWith(File.separator)) {
classFile = classFile.substring(1);
}
classes.add(classFile.replace(File.separator, "."));
}
} else {
JarFile jarFile = new JarFile(file);
if (url.getFile().endsWith(".jar")) {
Enumeration<JarEntry> entries = jarFile.entries();
while (entries.hasMoreElements()) {
JarEntry jarEntry = entries.nextElement();
if (jarEntry.getName().endsWith(".class")) {
classes.add(jarEntry.getName().replace(".class", "").replace("/", "."));
}
}
}
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
} catch (URISyntaxException e) {
e.printStackTrace();
}
}
return classes;
}
enter link description hereService Manager in java will get all implementing classes for an interface in J