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Compile Java code in-memory [duplicate]

This question already has answers here:
Compile code fully in memory with javax.tools.JavaCompiler [duplicate]
(7 answers)
Closed 6 years ago.
I want to treat a String as a Java file then compile and run it. In other words, use Java as a script language.
To get better performance, we should avoid writing .class files to disk.

This answer is from one of my blogs, Compile and Run Java Source Code in Memory.
Here are the three source code files.
MemoryJavaCompiler.java
package me.soulmachine.compiler;
import java.io.IOException;
import java.io.PrintWriter;
import java.io.Writer;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import javax.tools.*;
/**
* Simple interface to Java compiler using JSR 199 Compiler API.
*/
public class MemoryJavaCompiler {
private javax.tools.JavaCompiler tool;
private StandardJavaFileManager stdManager;
public MemoryJavaCompiler() {
tool = ToolProvider.getSystemJavaCompiler();
if (tool == null) {
throw new RuntimeException("Could not get Java compiler. Please, ensure that JDK is used instead of JRE.");
}
stdManager = tool.getStandardFileManager(null, null, null);
}
/**
* Compile a single static method.
*/
public Method compileStaticMethod(final String methodName, final String className,
final String source)
throws ClassNotFoundException {
final Map<String, byte[]> classBytes = compile(className + ".java", source);
final MemoryClassLoader classLoader = new MemoryClassLoader(classBytes);
final Class clazz = classLoader.loadClass(className);
final Method[] methods = clazz.getDeclaredMethods();
for (final Method method : methods) {
if (method.getName().equals(methodName)) {
if (!method.isAccessible()) method.setAccessible(true);
return method;
}
}
throw new NoSuchMethodError(methodName);
}
public Map<String, byte[]> compile(String fileName, String source) {
return compile(fileName, source, new PrintWriter(System.err), null, null);
}
/**
* compile given String source and return bytecodes as a Map.
*
* #param fileName source fileName to be used for error messages etc.
* #param source Java source as String
* #param err error writer where diagnostic messages are written
* #param sourcePath location of additional .java source files
* #param classPath location of additional .class files
*/
private Map<String, byte[]> compile(String fileName, String source,
Writer err, String sourcePath, String classPath) {
// to collect errors, warnings etc.
DiagnosticCollector<JavaFileObject> diagnostics =
new DiagnosticCollector<JavaFileObject>();
// create a new memory JavaFileManager
MemoryJavaFileManager fileManager = new MemoryJavaFileManager(stdManager);
// prepare the compilation unit
List<JavaFileObject> compUnits = new ArrayList<JavaFileObject>(1);
compUnits.add(fileManager.makeStringSource(fileName, source));
return compile(compUnits, fileManager, err, sourcePath, classPath);
}
private Map<String, byte[]> compile(final List<JavaFileObject> compUnits,
final MemoryJavaFileManager fileManager,
Writer err, String sourcePath, String classPath) {
// to collect errors, warnings etc.
DiagnosticCollector<JavaFileObject> diagnostics =
new DiagnosticCollector<JavaFileObject>();
// javac options
List<String> options = new ArrayList<String>();
options.add("-Xlint:all");
// options.add("-g:none");
options.add("-deprecation");
if (sourcePath != null) {
options.add("-sourcepath");
options.add(sourcePath);
}
if (classPath != null) {
options.add("-classpath");
options.add(classPath);
}
// create a compilation task
javax.tools.JavaCompiler.CompilationTask task =
tool.getTask(err, fileManager, diagnostics,
options, null, compUnits);
if (task.call() == false) {
PrintWriter perr = new PrintWriter(err);
for (Diagnostic diagnostic : diagnostics.getDiagnostics()) {
perr.println(diagnostic);
}
perr.flush();
return null;
}
Map<String, byte[]> classBytes = fileManager.getClassBytes();
try {
fileManager.close();
} catch (IOException exp) {
}
return classBytes;
}
}
MemoryJavaFileManager.java
package me.soulmachine.compiler;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FilterOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.URI;
import java.nio.CharBuffer;
import java.util.HashMap;
import java.util.Map;
import javax.tools.FileObject;
import javax.tools.ForwardingJavaFileManager;
import javax.tools.JavaFileManager;
import javax.tools.JavaFileObject;
import javax.tools.JavaFileObject.Kind;
import javax.tools.SimpleJavaFileObject;
/**
* JavaFileManager that keeps compiled .class bytes in memory.
*/
#SuppressWarnings("unchecked")
final class MemoryJavaFileManager extends ForwardingJavaFileManager {
/** Java source file extension. */
private final static String EXT = ".java";
private Map<String, byte[]> classBytes;
public MemoryJavaFileManager(JavaFileManager fileManager) {
super(fileManager);
classBytes = new HashMap<>();
}
public Map<String, byte[]> getClassBytes() {
return classBytes;
}
public void close() throws IOException {
classBytes = null;
}
public void flush() throws IOException {
}
/**
* A file object used to represent Java source coming from a string.
*/
private static class StringInputBuffer extends SimpleJavaFileObject {
final String code;
StringInputBuffer(String fileName, String code) {
super(toURI(fileName), Kind.SOURCE);
this.code = code;
}
public CharBuffer getCharContent(boolean ignoreEncodingErrors) {
return CharBuffer.wrap(code);
}
}
/**
* A file object that stores Java bytecode into the classBytes map.
*/
private class ClassOutputBuffer extends SimpleJavaFileObject {
private String name;
ClassOutputBuffer(String name) {
super(toURI(name), Kind.CLASS);
this.name = name;
}
public OutputStream openOutputStream() {
return new FilterOutputStream(new ByteArrayOutputStream()) {
public void close() throws IOException {
out.close();
ByteArrayOutputStream bos = (ByteArrayOutputStream)out;
classBytes.put(name, bos.toByteArray());
}
};
}
}
public JavaFileObject getJavaFileForOutput(JavaFileManager.Location location,
String className,
Kind kind,
FileObject sibling) throws IOException {
if (kind == Kind.CLASS) {
return new ClassOutputBuffer(className);
} else {
return super.getJavaFileForOutput(location, className, kind, sibling);
}
}
static JavaFileObject makeStringSource(String fileName, String code) {
return new StringInputBuffer(fileName, code);
}
static URI toURI(String name) {
File file = new File(name);
if (file.exists()) {
return file.toURI();
} else {
try {
final StringBuilder newUri = new StringBuilder();
newUri.append("mfm:///");
newUri.append(name.replace('.', '/'));
if(name.endsWith(EXT)) newUri.replace(newUri.length() - EXT.length(), newUri.length(), EXT);
return URI.create(newUri.toString());
} catch (Exception exp) {
return URI.create("mfm:///com/sun/script/java/java_source");
}
}
}
}
MemoryClassLoader.java
package me.soulmachine.compiler;
import java.io.File;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.StringTokenizer;
/**
* ClassLoader that loads .class bytes from memory.
*/
final class MemoryClassLoader extends URLClassLoader {
private Map<String, byte[]> classBytes;
public MemoryClassLoader(Map<String, byte[]> classBytes,
String classPath, ClassLoader parent) {
super(toURLs(classPath), parent);
this.classBytes = classBytes;
}
public MemoryClassLoader(Map<String, byte[]> classBytes, String classPath) {
this(classBytes, classPath, ClassLoader.getSystemClassLoader());
}
public MemoryClassLoader(Map<String, byte[]> classBytes) {
this(classBytes, null, ClassLoader.getSystemClassLoader());
}
public Class load(String className) throws ClassNotFoundException {
return loadClass(className);
}
public Iterable<Class> loadAll() throws ClassNotFoundException {
List<Class> classes = new ArrayList<Class>(classBytes.size());
for (String name : classBytes.keySet()) {
classes.add(loadClass(name));
}
return classes;
}
protected Class findClass(String className) throws ClassNotFoundException {
byte[] buf = classBytes.get(className);
if (buf != null) {
// clear the bytes in map -- we don't need it anymore
classBytes.put(className, null);
return defineClass(className, buf, 0, buf.length);
} else {
return super.findClass(className);
}
}
private static URL[] toURLs(String classPath) {
if (classPath == null) {
return new URL[0];
}
List<URL> list = new ArrayList<URL>();
StringTokenizer st = new StringTokenizer(classPath, File.pathSeparator);
while (st.hasMoreTokens()) {
String token = st.nextToken();
File file = new File(token);
if (file.exists()) {
try {
list.add(file.toURI().toURL());
} catch (MalformedURLException mue) {}
} else {
try {
list.add(new URL(token));
} catch (MalformedURLException mue) {}
}
}
URL[] res = new URL[list.size()];
list.toArray(res);
return res;
}
}
Explanations:
In order to represent a Java source file in memory instead of disk, I defined a StringInputBuffer class in the MemoryJavaFileManager.java.
To save the compiled .class files in memory, I implemented a class MemoryJavaFileManager. The main idea is to override the function getJavaFileForOutput() to store bytecodes into a map.
To load the bytecodes in memory, I have to implement a customized classloader MemoryClassLoader, which reads bytecodes in the map and turn them into classes.
Here is a unite test.
package me.soulmachine.compiler;
import org.junit.Test;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.ArrayList;
import static org.junit.Assert.assertEquals;
public class MemoryJavaCompilerTest {
private final static MemoryJavaCompiler compiler = new MemoryJavaCompiler();
#Test public void compileStaticMethodTest()
throws ClassNotFoundException, InvocationTargetException, IllegalAccessException {
final String source = "public final class Solution {\n"
+ "public static String greeting(String name) {\n"
+ "\treturn \"Hello \" + name;\n" + "}\n}\n";
final Method greeting = compiler.compileStaticMethod("greeting", "Solution", source);
final Object result = greeting.invoke(null, "soulmachine");
assertEquals("Hello soulmachine", result.toString());
}
}
Reference
JavaCompiler.java from Cloudera Morphlines
How to create an object from a string in Java (how to eval a string)?
InMemoryJavaCompiler
Java-Runtime-Compiler
动态的Java - 无废话JavaCompilerAPI中文指南

How to print class and interface name from a java jar file

I have a Jar in java which is containing 2 classes and 1 Interface. How can i get the interface and class names from the jar. Currently I am able to get the class names, but not the interface name.
List jClasses = getClasseNames("D://Test.jar");
System.out.println(jClasses.size());
for (int i = 0; i < jClasses.size(); i++) {
System.out.println("Print Classes ::" + jClasses.get(i));
if(( null != jClasses.getClass().getInterfaces()[i])) {
System.out.println(jClasses.getClass().getInterfaces()[i]);
} else {
System.out.println("No connection");
}
}
public static List getClasseNames(String jarName) {
ArrayList classes = new ArrayList();
try {
JarInputStream jarFile = new JarInputStream(new FileInputStream(
jarName));
JarEntry jarEntry;
while (true) {
jarEntry = jarFile.getNextJarEntry();
if (jarEntry == null) {
break;
}
if (jarEntry.getName().endsWith(".class")) {
classes.add(jarEntry.getName().replaceAll("/", "\\."));
}
}
} catch (Exception e) {
e.printStackTrace();
}
return classes;
}
output :
Print Classes ::com.java.testclient.PTest1.class
interface java.util.List
======
Print Classes ::com.java.testclient.ClassSpy.class
interface java.util.RandomAccess
======
Print Classes ::com.java.testclient.myInt.class
interface java.lang.Cloneable
======
Print Classes ::com.java.testclient.PTest.class
interface java.io.Serializable
Please suggest.

You can use this class:
package io.github.gabrielbb.java.utilities;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.ArrayList;
import java.util.List;
import java.util.jar.JarEntry;
import java.util.jar.JarInputStream;
/**
* #author Gabriel Basilio Brito
* #since 12/26/2016
* #version 1.1
*/
public class ClassesAndInterfacesFromJar {
public static List<Class> getJarClasses(String jarPath) throws IOException, ClassNotFoundException {
File jarFile = new File(jarPath);
return getJarClasses(jarFile);
}
public static List<Class> getJarClasses(File jar) throws IOException, ClassNotFoundException {
ArrayList<Class> classes = new ArrayList();
JarInputStream jarInputStream = null;
URLClassLoader cl;
try {
cl = URLClassLoader.newInstance(new URL[]{new URL("jar:file:" + jar + "!/")}); // To load classes inside the jar, after getting their names
jarInputStream = new JarInputStream(new FileInputStream(
jar)); // Getting a JarInputStream to iterate through the Jar files
JarEntry jarEntry = jarInputStream.getNextJarEntry();
while (jarEntry != null) {
if (jarEntry.getName().endsWith(".class")) { // Avoiding non ".class" files
String className = jarEntry.getName().replaceAll("/", "\\."); // The ClassLoader works with "." instead of "/"
className = className.substring(0, jarEntry.getName().length() - 6); // Removing ".class" from the string
Class clazz = cl.loadClass(className); // Loading the class by its name
classes.add(clazz);
}
jarEntry = jarInputStream.getNextJarEntry(); // Next File
}
} finally {
if (jarInputStream != null) {
jarInputStream.close(); // Closes the FileInputStream
}
}
return classes;
}
// Main Method for testing purposes
public static void main(String[] args) {
try {
String jarPath = "C://Test.jar";
List<Class> classes = getJarClasses(jarPath);
for (Class c : classes) {
// Here we can use the "isInterface" method to differentiate an Interface from a Class
System.out.println(c.isInterface() ? "Interface: " + c.getName() : "Class: " + c.getName());
}
} catch (Exception ex) {
System.err.println(ex);
}
}
It can be found at:
https://github.com/GabrielBB/Java-Utilities/blob/master/ClassesAndInterfacesFromJar.java

Library for processing Annotations

I've been trying to search the internet, but it seems I cannot find a library for helping processing of Annotations in a POJO. Is there any that exist?
Currently we can process this through code like this:
// Get id
Object id = null;
for (Field field : obj.getClass().getDeclaredFields()){
String fieldName = field.getName();
Object fieldValue = field.get(obj);
if (field.isAnnotationPresent(Id.class)){
id = fieldValue;
}
}
Is there a library to help quickly process annotation and with the associated value.

Take a look at How do I read all classes from a Java package in the classpath?
I was using https://code.google.com/p/reflections/, and now switched to this
package com.clemble.test.reflection;
import java.io.File;
import java.io.IOException;
import java.lang.annotation.Annotation;
import java.lang.annotation.ElementType;
import java.lang.annotation.Target;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.net.JarURLConnection;
import java.net.URI;
import java.net.URISyntaxException;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;
import java.util.jar.JarEntry;
import java.util.jar.JarFile;
public class AnnotationReflectionUtils {
/** URL prefix for loading from the file system: "file:" */
public static final String FILE_URL_PREFIX = "file:";
/** URL protocol for an entry from a jar file: "jar" */
public static final String URL_PROTOCOL_JAR = "jar";
/** URL protocol for an entry from a zip file: "zip" */
public static final String URL_PROTOCOL_ZIP = "zip";
/** URL protocol for an entry from a JBoss jar file: "vfszip" */
public static final String URL_PROTOCOL_VFSZIP = "vfszip";
/** URL protocol for a JBoss VFS resource: "vfs" */
public static final String URL_PROTOCOL_VFS = "vfs";
/** URL protocol for an entry from a WebSphere jar file: "wsjar" */
public static final String URL_PROTOCOL_WSJAR = "wsjar";
/** URL protocol for an entry from an OC4J jar file: "code-source" */
public static final String URL_PROTOCOL_CODE_SOURCE = "code-source";
/** Separator between JAR URL and file path within the JAR */
public static final String JAR_URL_SEPARATOR = "!/";
// Taken from https://stackoverflow.com/questions/1456930/how-do-i-read-all-classes-from-a-java-package-in-the-classpath
public static <T extends Annotation> List<Class<?>> findCandidates(String basePackage, Class<T> searchedAnnotation) {
ArrayList<Class<?>> candidates = new ArrayList<Class<?>>();
Enumeration<URL> urls;
String basePath = basePackage.replaceAll("\\.", File.separator);
try {
urls = Thread.currentThread().getContextClassLoader().getResources(basePath);
} catch (IOException e) {
throw new RuntimeException(e);
}
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
if (isJarURL(url)) {
try {
candidates.addAll(doFindPathMatchingJarResources(url, basePath, searchedAnnotation));
} catch (IOException e) {
throw new RuntimeException(e);
}
} else {
File directory = new File(url.getFile());
if (directory.exists() && directory.isDirectory()) {
for (File file : new File(url.getFile()).listFiles())
fetchCandidates(basePackage, file, searchedAnnotation, candidates);
}
}
}
return candidates;
}
private static <T extends Annotation> void fetchCandidates(String basePackage, File candidate, Class<T> searchedAnnotation, List<Class<?>> candidates) {
if (candidate.isDirectory()) {
for (File file : candidate.listFiles())
fetchCandidates(basePackage + "." + candidate.getName(), file, searchedAnnotation, candidates);
} else {
String fileName = candidate.getName();
if (fileName.endsWith(".class")) {
String className = fileName.substring(0, fileName.length() - 6);
Class<?> foundClass = checkCandidate(basePackage + "." + className, searchedAnnotation);
if (foundClass != null)
candidates.add(foundClass);
}
}
}
public static boolean isJarURL(URL url) {
String protocol = url.getProtocol();
return (URL_PROTOCOL_JAR.equals(protocol) || URL_PROTOCOL_ZIP.equals(protocol) || URL_PROTOCOL_WSJAR.equals(protocol) || (URL_PROTOCOL_CODE_SOURCE
.equals(protocol) && url.getPath().contains(JAR_URL_SEPARATOR)));
}
public static <T extends Annotation> Class<?> checkCandidate(String className, Class<T> searchedAnnotation) {
try {
Class<?> candidateClass = Class.forName(className);
Target target = searchedAnnotation.getAnnotation(Target.class);
for(ElementType elementType: target.value()) {
switch(elementType) {
case TYPE:
if (candidateClass.getAnnotation(searchedAnnotation) != null)
return candidateClass;
break;
case CONSTRUCTOR:
for(Constructor<?> constructor: candidateClass.getConstructors())
if(constructor.getAnnotation(searchedAnnotation) != null)
return candidateClass;
break;
case METHOD:
for(Method method: candidateClass.getMethods())
if(method.getAnnotation(searchedAnnotation) != null)
return candidateClass;
break;
case FIELD:
for(Field field: candidateClass.getFields())
if(field.getAnnotation(searchedAnnotation) != null)
return candidateClass;
break;
default:
break;
}
}
} catch (ClassNotFoundException e) {
} catch (NoClassDefFoundError e) {
}
return null;
}
/**
* Find all resources in jar files that match the given location pattern
* via the Ant-style PathMatcher.
*
* #param rootDirResource the root directory as Resource
* #param subPattern the sub pattern to match (below the root directory)
* #return the Set of matching Resource instances
* #throws IOException in case of I/O errors
* #see java.net.JarURLConnection
* #see org.springframework.util.PathMatcher
*/
protected static <T extends Annotation> Set<Class<?>> doFindPathMatchingJarResources(URL sourceUrl, String basePackage, Class<T> searchedAnnotation)
throws IOException {
URLConnection con = sourceUrl.openConnection();
JarFile jarFile;
String jarFileUrl;
String rootEntryPath;
boolean newJarFile = false;
if (con instanceof JarURLConnection) {
// Should usually be the case for traditional JAR files.
JarURLConnection jarCon = (JarURLConnection) con;
jarFile = jarCon.getJarFile();
jarFileUrl = jarCon.getJarFileURL().toExternalForm();
JarEntry jarEntry = jarCon.getJarEntry();
rootEntryPath = (jarEntry != null ? jarEntry.getName() : "");
} else {
// No JarURLConnection -> need to resort to URL file parsing.
// We'll assume URLs of the format "jar:path!/entry", with the protocol
// being arbitrary as long as following the entry format.
// We'll also handle paths with and without leading "file:" prefix.
String urlFile = sourceUrl.getFile();
int separatorIndex = urlFile.indexOf(JAR_URL_SEPARATOR);
if (separatorIndex != -1) {
jarFileUrl = urlFile.substring(0, separatorIndex);
rootEntryPath = urlFile.substring(separatorIndex + JAR_URL_SEPARATOR.length());
jarFile = getJarFile(jarFileUrl);
} else {
jarFile = new JarFile(urlFile);
jarFileUrl = urlFile;
rootEntryPath = "";
}
newJarFile = true;
}
try {
if (!"".equals(rootEntryPath) && !rootEntryPath.endsWith("/")) {
// Root entry path must end with slash to allow for proper matching.
// The Sun JRE does not return a slash here, but BEA JRockit does.
rootEntryPath = rootEntryPath + "/";
}
Set<Class<?>> result = new LinkedHashSet<Class<?>>(8);
for (Enumeration<JarEntry> entries = jarFile.entries(); entries.hasMoreElements();) {
JarEntry entry = entries.nextElement();
String entryPath = entry.getName();
if (entryPath.startsWith(rootEntryPath) && entryPath.endsWith(".class")) {
int entryLength = entryPath.length();
String className = entryPath.replaceAll(File.separator, ".").substring(0, entryLength - 6);
Class<?> foundClass = checkCandidate(className, searchedAnnotation);
if (foundClass != null)
result.add(foundClass);
}
}
return result;
} finally {
// Close jar file, but only if freshly obtained -
// not from JarURLConnection, which might cache the file reference.
if (newJarFile) {
jarFile.close();
}
}
}
/**
* Resolve the given jar file URL into a JarFile object.
*/
protected static JarFile getJarFile(String jarFileUrl) throws IOException {
if (jarFileUrl.startsWith(FILE_URL_PREFIX)) {
try {
return new JarFile(new URI(jarFileUrl.replaceAll(" ", "%20")).getSchemeSpecificPart());
} catch (URISyntaxException ex) {
// Fallback for URLs that are not valid URIs (should hardly ever happen).
return new JarFile(jarFileUrl.substring(FILE_URL_PREFIX.length()));
}
} else {
return new JarFile(jarFileUrl);
}
}
}
This is based on some Spring utility, class which I could not use directly in my application, but I forgot which one was it.

Why can reflection package scanning work in standard JVM and not work in Android

I've written simple ReflectionUtils, to find all classes with specific Annotation, I am using it successfully on my server, but for some reason it does not perform as expected on Android. (I specifically use it to find all classes annotated with #JsonTypeName, and add them to ObjectMapper context)
What might be the problem?
package com.acme.reflection.utils;
import java.io.File;
import java.io.IOException;
import java.lang.annotation.Annotation;
import java.lang.annotation.ElementType;
import java.lang.annotation.Target;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.net.JarURLConnection;
import java.net.URI;
import java.net.URISyntaxException;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;
import java.util.jar.JarEntry;
import java.util.jar.JarFile;
public class ReflectionUtils {
/** URL prefix for loading from the file system: "file:" */
public static final String FILE_URL_PREFIX = "file:";
/** URL protocol for an entry from a jar file: "jar" */
public static final String URL_PROTOCOL_JAR = "jar";
/** URL protocol for an entry from a zip file: "zip" */
public static final String URL_PROTOCOL_ZIP = "zip";
/** URL protocol for an entry from a JBoss jar file: "vfszip" */
public static final String URL_PROTOCOL_VFSZIP = "vfszip";
/** URL protocol for a JBoss VFS resource: "vfs" */
public static final String URL_PROTOCOL_VFS = "vfs";
/** URL protocol for an entry from a WebSphere jar file: "wsjar" */
public static final String URL_PROTOCOL_WSJAR = "wsjar";
/** URL protocol for an entry from an OC4J jar file: "code-source" */
public static final String URL_PROTOCOL_CODE_SOURCE = "code-source";
/** Separator between JAR URL and file path within the JAR */
public static final String JAR_URL_SEPARATOR = "!/";
// Taken from http://stackoverflow.com/questions/1456930/how-do-i-read-all-classes-from-a-java-package-in-the-classpath
public static <T extends Annotation> List<Class<?>> findCandidates(String basePackage, Class<T> searchedAnnotation) {
ArrayList<Class<?>> candidates = new ArrayList<Class<?>>();
Enumeration<URL> urls;
String basePath = basePackage.replaceAll("\\.", File.separator);
try {
urls = Thread.currentThread().getContextClassLoader().getResources(basePath);
} catch (IOException e) {
throw new RuntimeException(e);
}
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
if (isJarURL(url)) {
try {
candidates.addAll(doFindPathMatchingJarResources(url, basePath, searchedAnnotation));
} catch (IOException e) {
throw new RuntimeException(e);
}
} else {
File directory = new File(url.getFile());
if (directory.exists() && directory.isDirectory()) {
for (File file : new File(url.getFile()).listFiles())
fetchCandidates(basePackage, file, searchedAnnotation, candidates);
}
}
}
return candidates;
}
private static <T extends Annotation> void fetchCandidates(String basePackage, File candidate, Class<T> searchedAnnotation, List<Class<?>> candidates) {
if (candidate.isDirectory()) {
for (File file : candidate.listFiles())
fetchCandidates(basePackage + "." + candidate.getName(), file, searchedAnnotation, candidates);
} else {
String fileName = candidate.getName();
if (fileName.endsWith(".class")) {
String className = fileName.substring(0, fileName.length() - 6);
Class<?> foundClass = checkCandidate(basePackage + "." + className, searchedAnnotation);
if (foundClass != null)
candidates.add(foundClass);
}
}
}
public static boolean isJarURL(URL url) {
String protocol = url.getProtocol();
return (URL_PROTOCOL_JAR.equals(protocol) || URL_PROTOCOL_ZIP.equals(protocol) || URL_PROTOCOL_WSJAR.equals(protocol) || (URL_PROTOCOL_CODE_SOURCE
.equals(protocol) && url.getPath().contains(JAR_URL_SEPARATOR)));
}
public static <T extends Annotation> Class<?> checkCandidate(String className, Class<T> searchedAnnotation) {
try {
Class<?> candidateClass = Class.forName(className);
Target target = searchedAnnotation.getAnnotation(Target.class);
for(ElementType elementType: target.value()) {
switch(elementType) {
case TYPE:
if (candidateClass.getAnnotation(searchedAnnotation) != null)
return candidateClass;
break;
case CONSTRUCTOR:
for(Constructor<?> constructor: candidateClass.getConstructors())
if(constructor.getAnnotation(searchedAnnotation) != null)
return candidateClass;
break;
case METHOD:
for(Method method: candidateClass.getMethods())
if(method.getAnnotation(searchedAnnotation) != null)
return candidateClass;
break;
case FIELD:
for(Field field: candidateClass.getFields())
if(field.getAnnotation(searchedAnnotation) != null)
return candidateClass;
break;
default:
break;
}
}
} catch (ClassNotFoundException | NoClassDefFoundError e) {
;
}
return null;
}
/**
* Find all resources in jar files that match the given location pattern
* via the Ant-style PathMatcher.
*
* #param rootDirResource the root directory as Resource
* #param subPattern the sub pattern to match (below the root directory)
* #return the Set of matching Resource instances
* #throws IOException in case of I/O errors
* #see java.net.JarURLConnection
* #see org.springframework.util.PathMatcher
*/
protected static <T extends Annotation> Set<Class<?>> doFindPathMatchingJarResources(URL sourceUrl, String basePackage, Class<T> searchedAnnotation)
throws IOException {
URLConnection con = sourceUrl.openConnection();
JarFile jarFile;
String jarFileUrl;
String rootEntryPath;
boolean newJarFile = false;
if (con instanceof JarURLConnection) {
// Should usually be the case for traditional JAR files.
JarURLConnection jarCon = (JarURLConnection) con;
jarFile = jarCon.getJarFile();
jarFileUrl = jarCon.getJarFileURL().toExternalForm();
JarEntry jarEntry = jarCon.getJarEntry();
rootEntryPath = (jarEntry != null ? jarEntry.getName() : "");
} else {
// No JarURLConnection -> need to resort to URL file parsing.
// We'll assume URLs of the format "jar:path!/entry", with the protocol
// being arbitrary as long as following the entry format.
// We'll also handle paths with and without leading "file:" prefix.
String urlFile = sourceUrl.getFile();
int separatorIndex = urlFile.indexOf(JAR_URL_SEPARATOR);
if (separatorIndex != -1) {
jarFileUrl = urlFile.substring(0, separatorIndex);
rootEntryPath = urlFile.substring(separatorIndex + JAR_URL_SEPARATOR.length());
jarFile = getJarFile(jarFileUrl);
} else {
jarFile = new JarFile(urlFile);
jarFileUrl = urlFile;
rootEntryPath = "";
}
newJarFile = true;
}
try {
if (!"".equals(rootEntryPath) && !rootEntryPath.endsWith("/")) {
// Root entry path must end with slash to allow for proper matching.
// The Sun JRE does not return a slash here, but BEA JRockit does.
rootEntryPath = rootEntryPath + "/";
}
Set<Class<?>> result = new LinkedHashSet<>(8);
for (Enumeration<JarEntry> entries = jarFile.entries(); entries.hasMoreElements();) {
JarEntry entry = entries.nextElement();
String entryPath = entry.getName();
if (entryPath.startsWith(rootEntryPath) && entryPath.endsWith(".class")) {
int entryLength = entryPath.length();
String className = entryPath.replaceAll(File.separator, ".").substring(0, entryLength - 6);
Class<?> foundClass = checkCandidate(className, searchedAnnotation);
if (foundClass != null)
result.add(foundClass);
}
}
return result;
} finally {
// Close jar file, but only if freshly obtained -
// not from JarURLConnection, which might cache the file reference.
if (newJarFile) {
jarFile.close();
}
}
}
/**
* Resolve the given jar file URL into a JarFile object.
*/
protected static JarFile getJarFile(String jarFileUrl) throws IOException {
if (jarFileUrl.startsWith(FILE_URL_PREFIX)) {
try {
return new JarFile(new URI(jarFileUrl.replaceAll(" ", "%20")).getSchemeSpecificPart());
} catch (URISyntaxException ex) {
// Fallback for URLs that are not valid URIs (should hardly ever happen).
return new JarFile(jarFileUrl.substring(FILE_URL_PREFIX.length()));
}
} else {
return new JarFile(jarFileUrl);
}
}
}

Found a simmilar question on:
Implementing Spring-like package scanning in Android
After some consideration, I decided to change the approach for ObjectManager.
I keep module configurations in predefined package xxx.yyy.zzz.json.AAAJsonModule and on ObjectMapper construction try to load module configurations in xxx.yyy.zzz.json.{AAA}JsonModule modules, if module is missing, I ignore it. So that way I can dynamically change ObjectMapper mapping, based on the present jars in classpath.

Run a simple text file as Java

I have a simple .txt file which has pure Java code inside it like
public class C {
public static void main(String[] args ) {
System.out.println("This is executed");
}
}
The file is named C.txt. Now I want to write Java code that will read the code in C.txt and will compile and run the read code as a pure Java file. Note, I can easily rename C.txt to C.java and compile and run the code manually. However, this is not my intention. I want to read the .txt file as is and execute the code directly. Is this possible somehow?

You can use the javax.tools api form Java 6 to compile the code on the fly. However since your extension is illegal it will complain with a error: C.txt Class names are only accepted if annotation processing is explicitly requested.
To get around this (as mentioned in the comments) you must first load the code into a String and then execute it:
import javax.tools.JavaCompiler;
import javax.tools.JavaFileObject;
import javax.tools.SimpleJavaFileObject;
import javax.tools.ToolProvider;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.lang.reflect.Method;
import java.net.URI;
import java.util.Iterator;
import java.util.NoSuchElementException;
public class MyCompiler2 {
public static void main(String[] args) throws Exception {
String program = "";
try {
BufferedReader in = new BufferedReader(new FileReader("C.txt"));
String str;
while ((str = in.readLine()) != null) {
program += str;
}
in.close();
} catch (IOException e) {
}
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
Iterable<? extends JavaFileObject> fileObjects;
fileObjects = getJavaSourceFromString(program);
compiler.getTask(null, null, null, null, null, fileObjects).call();
Class<?> clazz = Class.forName("C");
Method m = clazz.getMethod("main", new Class[]{String[].class});
Object[] _args = new Object[]{new String[0]};
m.invoke(null, _args);
}
static Iterable<JavaSourceFromString> getJavaSourceFromString(String code) {
final JavaSourceFromString jsfs;
jsfs = new JavaSourceFromString("code", code);
return new Iterable<JavaSourceFromString>() {
public Iterator<JavaSourceFromString> iterator() {
return new Iterator<JavaSourceFromString>() {
boolean isNext = true;
public boolean hasNext() {
return isNext;
}
public JavaSourceFromString next() {
if (!isNext)
throw new NoSuchElementException();
isNext = false;
return jsfs;
}
public void remove() {
throw new UnsupportedOperationException();
}
};
}
};
}
}
class JavaSourceFromString extends SimpleJavaFileObject {
final String code;
JavaSourceFromString(String name, String code) {
super(URI.create("string:///" + name.replace('.', '/') + Kind.SOURCE.extension), Kind.SOURCE);
this.code = code;
}
public CharSequence getCharContent(boolean ignoreEncodingErrors) {
return code;
}
}
Notice how you need to explicitly provide the method and class name in order for reflection to execute your code.

I think I'd start with BeanShell, which allows you to compile and execute Java source held in a string.

Check out this thread for how to start the compile from within Java...
How to set the source for compilation by a CompilationTask