Java 内存级动态编译

先定义接口：

package Test;
public interface Printer {
public void print();
}

然后如下：
package Test;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.URI;
import java.util.Arrays;
import javax.tools.FileObject;
import javax.tools.ForwardingJavaFileManager;
import javax.tools.JavaCompiler;
import javax.tools.JavaCompiler.CompilationTask;
import javax.tools.JavaFileManager;
import javax.tools.JavaFileObject;
import javax.tools.SimpleJavaFileObject;
import javax.tools.JavaFileObject.Kind;
import javax.tools.ToolProvider;

public class MyTest {
//自定义JavaSourceFromString，作为源代码的抽象文件
static class JavaSourceFromString extends SimpleJavaFileObject{
final String code ;
public JavaSourceFromString(String name,String code) {
super(URI.create("string:///"+name.replace('.', '/')+Kind.SOURCE.extension), Kind.SOURCE);
this.code = code;
}
@Override
public CharSequence getCharContent(boolean ignoreEncodingErrors){
return code ;
}

}
//JavaClassFileObject，代表class的文件抽象
static class JavaClassFileObject extends SimpleJavaFileObject{
ByteArrayOutputStream outputStream=null;
public JavaClassFileObject(String className, Kind kind) {
super(URI.create("string:///"+className.replace('.', '/')+kind.extension), kind);
outputStream = new ByteArrayOutputStream();
}
@Override
public OutputStream openOutputStream() throws IOException{
return this.outputStream;
}
public byte[] getClassBytes(){
return outputStream.toByteArray();
}
}
//ClassFileManager，修改JavaFileManager生成class的JavaFileObject的行为，另外返回一个自定义ClassLoader用于返回内存中的字节码对应的类模板
static class ClassFileManager extends ForwardingJavaFileManager<JavaFileManager>{
private JavaClassFileObject classFileObject;
protected ClassFileManager(JavaFileManager fileManager) {
super(fileManager);
}
@Override
public JavaFileObject getJavaFileForOutput(Location location,String className,Kind kind,FileObject sibling) throws IOException{
classFileObject = new JavaClassFileObject(className, kind);
return classFileObject;
}
@Override
public ClassLoader getClassLoader(Location location){
return new ClassLoader() {
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException{
byte[] classBytes = classFileObject.getClassBytes();
return super.defineClass(name, classBytes, 0, classBytes.length);
}
};
}
}
public static void main(String[] args) {
String string = "package Test; \n" + "public class MyPrint implements Printer{ \n" + "@Override \n"
+ "public void print() { \n" + "System.out.println(\"MyPrint...\"); \n" + "} \n" + "}";
//生成源代码的JavaFileObject
SimpleJavaFileObject fileObject = new JavaSourceFromString("Test.MyPrint", string);
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
//被修改后的JavaFileManager
JavaFileManager fileManager = new ClassFileManager(compiler.getStandardFileManager(null, null, null));
//执行编译
CompilationTask task = compiler.getTask(null, fileManager, null, null, null, Arrays.asList(fileObject));
task.call();
//获得ClassLoader，加载class文件
ClassLoader classLoader = fileManager.getClassLoader(null);
try {
Class<?> printClass = classLoader.loadClass("Test.MyPrint");
//获得实例
Printer printer = (Printer)printClass.newInstance();
printer.print();
} catch (Exception e) {
e.printStackTrace();
}
}
}