三、ASM
我們知道Java是靜態(tài)語言，而python、ruby是動態(tài)語言，Java程序一旦寫好很難在運行時更改類的行為，而python、ruby可以。
不過基于bytecode層面上我們可以做一些手腳，來使Java程序多一些靈活性和Magic，ASM就是這樣一個應(yīng)用廣泛的開源庫。
ASM is a Java bytecode manipulation framework. It can be used to dynamically generate stub classes or other proxy classes,
directly in binary form, or to dynamically modify classes at load time, i.e., just before they are loaded into the Java
Virtual Machine.
ASM完成了BCEL和SERP同樣的功能，但ASM
只有30多k，而后兩者分別是350k和150k。apache真是越來越過氣了。
讓我們來看一個ASM的簡單例子Helloworld.java，它生成一個Example類和一個main方法，main方法打印"Hello world!"語句:
- import java.io.FileOutputStream;
- import java.io.PrintStream;
-
- import org.objectweb.asm.ClassWriter;
- import org.objectweb.asm.MethodVisitor;
- import org.objectweb.asm.Opcodes;
- import org.objectweb.asm.Type;
- import org.objectweb.asm.commons.GeneratorAdapter;
- import org.objectweb.asm.commons.Method;
-
- public class Helloworld extends ClassLoader implements Opcodes {
-
- public static void main(final String args[]) throws Exception {
-
-
-
-
- ClassWriter cw = new ClassWriter(0);
- cw.visit(V1_1, ACC_PUBLIC, "Example", null, "java/lang/Object", null);
- MethodVisitor mw = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null,
- null);
- mw.visitVarInsn(ALOAD, 0);
- mw.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V");
- mw.visitInsn(RETURN);
- mw.visitMaxs(1, 1);
- mw.visitEnd();
- mw = cw.visitMethod(ACC_PUBLIC + ACC_STATIC, "main",
- "([Ljava/lang/String;)V", null, null);
- mw.visitFieldInsn(GETSTATIC, "java/lang/System", "out",
- "Ljava/io/PrintStream;");
- mw.visitLdcInsn("Hello world!");
- mw.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println",
- "(Ljava/lang/String;)V");
- mw.visitInsn(RETURN);
- mw.visitMaxs(2, 2);
- mw.visitEnd();
- byte[] code = cw.toByteArray();
- FileOutputStream fos = new FileOutputStream("Example.class");
- fos.write(code);
- fos.close();
- Helloworld loader = new Helloworld();
- Class exampleClass = loader
- .defineClass("Example", code, 0, code.length);
- exampleClass.getMethods()[0].invoke(null, new Object[] { null });
-
-
-
-
-
- cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
- cw.visit(V1_1, ACC_PUBLIC, "Example", null, "java/lang/Object", null);
- Method m = Method.getMethod("void <init> ()");
- GeneratorAdapter mg = new GeneratorAdapter(ACC_PUBLIC, m, null, null,
- cw);
- mg.loadThis();
- mg.invokeConstructor(Type.getType(Object.class), m);
- mg.returnValue();
- mg.endMethod();
- m = Method.getMethod("void main (String[])");
- mg = new GeneratorAdapter(ACC_PUBLIC + ACC_STATIC, m, null, null, cw);
- mg.getStatic(Type.getType(System.class), "out", Type
- .getType(PrintStream.class));
- mg.push("Hello world!");
- mg.invokeVirtual(Type.getType(PrintStream.class), Method
- .getMethod("void println (String)"));
- mg.returnValue();
- mg.endMethod();
- cw.visitEnd();
- code = cw.toByteArray();
- loader = new Helloworld();
- exampleClass = loader.defineClass("Example", code, 0, code.length);
- exampleClass.getMethods()[0].invoke(null, new Object[] { null });
- }
- }
我們看到上面的例子分別使用ASM的MethodVisitor和GeneratorAdapter兩種方式來動態(tài)生成Example類并調(diào)用打印語句。
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