Java动态编译Java代码，并加载到内存中然后执行类中方法

近来要开发一个上传java文件，就能动态的将其加载到内存中并执行它的方法的小功能，

在网上找到了一篇不错的api介绍，特将其记下，下面直接进入正题：

步骤：

1.编译

public static Map<String, byte[]> compile(String javaName, String javaSrc) {
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager stdManager = compiler.getStandardFileManager(null, null, null);
try (MemoryJavaFileManager manager = new MemoryJavaFileManager(stdManager)) {
JavaFileObject javaFileObject = manager.makeStringSource(javaName, javaSrc);
JavaCompiler.CompilationTask task = compiler.getTask(null, manager, null, null, null, Arrays.asList(javaFileObject));
if (task.call())
return manager.getClassBytes();
} catch (IOException e) {
e.printStackTrace();
}
return null;
}

该方法的功能描述：通过类名和其代码（Java代码字符串），编译得到字节码，返回类名及其对应类的字节码，封装于Map中，

值得注意的是，平常类中就编译出来的字节码只有一个类，但是考虑到内部类的情况，会出现很多个类名及其字节码，所以用Map封装方便。

参数详解：javaName,类名，一般就是我们上传Java文件的名字，javaSrc，Java代码的字符串

2.类的动态加载

public static class MemoryClassLoader extends URLClassLoader {
Map<String, byte[]> classBytes = new HashMap<String, byte[]>();
public MemoryClassLoader(Map<String, byte[]> classBytes) {
super(new URL[0], MemoryClassLoader.class.getClassLoader());
this.classBytes.putAll(classBytes);
}
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
byte[] buf = classBytes.get(name);
if (buf == null) {
return super.findClass(name);
}
classBytes.remove(name);
return defineClass(name, buf, 0, buf.length);
}
}

功能：根据类名和字节码，先根据类名在内存中查找是否已存在该类，若不存在则调用URLClassLoader 的

defineClass方法加载该类。

3.调用动态加载进来的类的方法

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Class clazz = classLoader.loadClass("TestClass");
Object object = clazz.newInstance();
Method method = clazz.getMethod("add", int.class, int.class);
Object returnValue = method.invoke(object, a, b);
}

其实不一定非得用这种方法，可以调用Class.forName()获取Class对象都行。

以下是一个小demo，来源：http://blog.sina.com.cn/s/blog_70279be20101dk0j.html

import javax.tools.*;
import java.io.*;
import java.net.URI;
import java.net.URL;
import java.net.URLClassLoader;
import java.nio.CharBuffer;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
public class DynamicLoader {
/**
* auto fill in the java-name with code, return null if cannot find the public class
* @param javaSrc source code string
* @return return the Map, the KEY means ClassName, the VALUE means bytecode.
*/
public static Map<String, byte[]> compile(String javaSrc) {
Pattern pattern = Pattern.compile("public\\s+class\\s+(\\w+)");
Matcher matcher = pattern.matcher(javaSrc);
if (matcher.find())
return compile(matcher.group(1) + ".java", javaSrc);
return null;
}
/**
* @param javaName the name of your public class,eg: <code>TestClass.java</code>
* @param javaSrc source code string
* @return return the Map, the KEY means ClassName, the VALUE means bytecode.
*/
public static Map<String, byte[]> compile(String javaName, String javaSrc) {
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager stdManager = compiler.getStandardFileManager(null, null, null);
try (MemoryJavaFileManager manager = new MemoryJavaFileManager(stdManager)) {
JavaFileObject javaFileObject = manager.makeStringSource(javaName, javaSrc);
JavaCompiler.CompilationTask task = compiler.getTask(null, manager, null, null, null, Arrays.asList(javaFileObject));
if (task.call())
return manager.getClassBytes();
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
public static class MemoryClassLoader extends URLClassLoader {
Map<String, byte[]> classBytes = new HashMap<String, byte[]>();
public MemoryClassLoader(Map<String, byte[]> classBytes) {
super(new URL[0], MemoryClassLoader.class.getClassLoader());
this.classBytes.putAll(classBytes);
}
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
byte[] buf = classBytes.get(name);
if (buf == null) {
return super.findClass(name);
}
classBytes.remove(name);
return defineClass(name, buf, 0, buf.length);
}
}
}
* MemoryJavaFileManager.java
* @author A. Sundararajan
*/
/**
* 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<String, byte[]>();
}
public Map<String, byte[]> getClassBytes() {
return classBytes;
}
public void close() throws IOException {
classBytes = new HashMap<String, byte[]>();
}
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 name, String code) {
super(toURI(name), Kind.SOURCE);
this.code = code;
}
public CharBuffer getCharContent(boolean ignoreEncodingErrors) {
return CharBuffer.wrap(code);
}
public Reader openReader() {
return new StringReader(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,
JavaFileObject.Kind kind,
FileObject sibling) throws IOException {
if (kind == JavaFileObject.Kind.CLASS) {
return new ClassOutputBuffer(className);
} else {
return super.getJavaFileForOutput(location, className, kind, sibling);
}
}
static JavaFileObject makeStringSource(String name, String code) {
return new StringInputBuffer(name, 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");
}
}
}
}

测试代码如下：

import org.junit.Assert;
import org.junit.Test;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Iterator;
import java.util.Map;
import java.util.Random;
public class DynamicLoaderTestCase {
private String javaSrc = "public class TestClass{" +
"public void sayHello(String msg) {" +
"System.out.printf(\"Hello %s! This message from a Java String.%n\",msg);" +
"}" +
"public int add(int a,int b){" +
"return a+b;" +
"}" +
"}";
@Test
public void testCompile() {
Map<String, byte[]> bytecode = DynamicLoader.compile("TestClass.java", javaSrc);
for (Iterator<String> iterator = bytecode.keySet().iterator(); iterator.hasNext(); ) {
String key = iterator.next();
byte[] code = bytecode.get(key);
System.out.printf("Class: %s, Length: %d%n", key, code.length);
}
// Since the compiler and compiler options are different, the size of the bytes may be inconsistent.
Assert.assertEquals(558, bytecode.get("TestClass").length);
}
@Test
public void testInvoke() throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {
Random random = new Random();
int a = random.nextInt(1024);
int b = random.nextInt(1024);
Map<String, byte[]> bytecode = DynamicLoader.compile("TestClass.java", javaSrc);
DynamicLoader.MemoryClassLoader classLoader = new DynamicLoader.MemoryClassLoader(bytecode);
Class clazz = classLoader.loadClass("TestClass");
Object object = clazz.newInstance();
Method method = clazz.getMethod("add", int.class, int.class);
Object returnValue = method.invoke(object, a, b);
Assert.assertEquals(a + b, returnValue);
}
}

在开发的过程中，我们将上传的Java文件动态编译，加载到Tomcat的容器中，因此不能用Java的classloader,

我们用到了WebappClassLoader

Java动态编译Java代码，并加载到内存中然后执行类中方法

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