class热替换项目hot2hot
纠错
28 Jan 2015
我们用eclipse+tomcat开发web项目时，经常会这样做，修改个java文件，然后不需要重启tomcat就可以直接看到修改后的结果， 这就是热替换技术。
对热替换技术原理的探索源于hotswap，但是hotswap的限制比较多。用户手册：http://jm-blog.aliapp.com/?p=641
阿里自己实现了hotcode2，功能挺强大的，而且在不断的更新。
原理就是：切入java.lang.ClassLoader中的方法defineClass，通过asm将加载的类修改。比如在方法中加入代码，用于判断当前class是否修改过，如果修改过，就重新加载。
下面贴一张hotswap和hotcode2的比较图：
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接下来简单说下hot2hot：
git地址：https://github.com/liuxinglanyue/hot2hot
首先通过javagent的方式切入， AgentMain类做了两件事儿。第一，修改java.lang.ClassLoader中的defineClass方法。第二，定时扫描一个文件夹，替换class。
defineClass方法如下：
protected final Class defineClass(String name, byte[] b, int off, int len,
ProtectionDomain protectionDomain)
throws ClassFormatError
{
//hot2hot
if(!name.startsWith("sun.") && !name.startsWith("java.") && !name.startsWith("javax.")
&& !name.startsWith("org.omg.") && !name.startsWith("org.w3c.") && !name.startsWith("org.xml.")
&& !name.startsWith("sun.")) {
protectionDomain = preDefineClass(name, protectionDomain);
Class c = null;
String source = defineClassSourceLocation(protectionDomain);
try {
c = defineClass1(name, b, off, len, protectionDomain, source);
} catch (ClassFormatError cf) {
c = defineTransformedClass(name, b, off, len, protectionDomain, cf,
source);
}
postDefineClass(c, protectionDomain);
Clazzs.setClazz(name, c);
return c;
}
//hot2hot
protectionDomain = preDefineClass(name, protectionDomain);
Class c = null;
String source = defineClassSourceLocation(protectionDomain);
try {
c = defineClass1(name, b, off, len, protectionDomain, source);
} catch (ClassFormatError cfe) {
c = defineTransformedClass(name, b, off, len, protectionDomain, cfe,
source);
}
postDefineClass(c, protectionDomain);
return c;
}
Clazzs内部就是ConcurrentHashMap，修改defineClass的目的就是将Class信息保存起来。
修改defineClass方法的代码如下：
package org.hot2hot.jdk;
import java.security.ProtectionDomain;
import org.hot2hot.utils.Clazzs;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.Label;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.Type;
/**
* 修改ClassLoader的defineClass
* @author jiaojianfeng
*
*/
public class ClassLoaderAdapter extends ClassVisitor {
public ClassLoaderAdapter(ClassVisitor cv){
super(Opcodes.ASM4, cv);
}
@Override
public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);
if (name.equals("defineClass")
&& desc.equals(Type.getMethodDescriptor(Type.getType(Class.class), Type.getType(String.class),
Type.getType(byte[].class), Type.INT_TYPE, Type.INT_TYPE,
Type.getType(ProtectionDomain.class)))) {
return new MethodVisitor(Opcodes.ASM4, mv) {
@Override
public void visitCode() {
super.visitCode();
//if 判断
Label jump = new Label();
Label tryLabel = new Label();
mv.visitVarInsn(Opcodes.ALOAD, 1);
mv.visitLdcInsn("com.sun.");
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(String.class),
"startsWith", "(Ljava/lang/String;)Z");
mv.visitJumpInsn(Opcodes.IFNE, jump);
mv.visitVarInsn(Opcodes.ALOAD, 1);
mv.visitLdcInsn("java.");
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(String.class),
"startsWith", "(Ljava/lang/String;)Z");
mv.visitJumpInsn(Opcodes.IFNE, jump);
mv.visitVarInsn(Opcodes.ALOAD, 1);
mv.visitLdcInsn("javax.");
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(String.class),
"startsWith", "(Ljava/lang/String;)Z");
mv.visitJumpInsn(Opcodes.IFNE, jump);
mv.visitVarInsn(Opcodes.ALOAD, 1);
mv.visitLdcInsn("org.omg.");
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(String.class),
"startsWith", "(Ljava/lang/String;)Z");
mv.visitJumpInsn(Opcodes.IFNE, jump);
mv.visitVarInsn(Opcodes.ALOAD, 1);
mv.visitLdcInsn("org.w3c.");
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(String.class),
"startsWith", "(Ljava/lang/String;)Z");
mv.visitJumpInsn(Opcodes.IFNE, jump);
mv.visitVarInsn(Opcodes.ALOAD, 1);
mv.visitLdcInsn("org.xml.");
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(String.class),
"startsWith", "(Ljava/lang/String;)Z");
mv.visitJumpInsn(Opcodes.IFNE, jump);
mv.visitVarInsn(Opcodes.ALOAD, 1);
mv.visitLdcInsn("sun.");
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(String.class),
"startsWith", "(Ljava/lang/String;)Z");
mv.visitJumpInsn(Opcodes.IFNE, jump);
//if
mv.visitVarInsn(Opcodes.ALOAD, 0);
mv.visitVarInsn(Opcodes.ALOAD, 1);
mv.visitVarInsn(Opcodes.ALOAD, 5);
mv.visitMethodInsn(Opcodes.INVOKESPECIAL, Type.getInternalName(ClassLoader.class),
"preDefineClass", "(Ljava/lang/String;Ljava/security/ProtectionDomain;)Ljava/security/ProtectionDomain;");
mv.visitVarInsn(Opcodes.ASTORE, 5);
mv.visitInsn(Opcodes.ACONST_NULL);
mv.visitVarInsn(Opcodes.ASTORE, 6);
mv.visitVarInsn(Opcodes.ALOAD, 0);
mv.visitVarInsn(Opcodes.ALOAD, 5);
mv.visitMethodInsn(Opcodes.INVOKESPECIAL, Type.getInternalName(ClassLoader.class),
"defineClassSourceLocation", "(Ljava/security/ProtectionDomain;)Ljava/lang/String;");
mv.visitVarInsn(Opcodes.ASTORE, 7);
mv.visitVarInsn(Opcodes.ALOAD, 0);
mv.visitVarInsn(Opcodes.ALOAD, 1);
mv.visitVarInsn(Opcodes.ALOAD, 2);
mv.visitVarInsn(Opcodes.ILOAD, 3);
mv.visitVarInsn(Opcodes.ILOAD, 4);
mv.visitVarInsn(Opcodes.ALOAD, 5);
mv.visitVarInsn(Opcodes.ALOAD, 7);
mv.visitMethodInsn(Opcodes.INVOKESPECIAL, Type.getInternalName(ClassLoader.class),
"defineClass1", "(Ljava/lang/String;[BIILjava/security/ProtectionDomain;Ljava/lang/String;)Ljava/lang/Class;");
mv.visitVarInsn(Opcodes.ASTORE, 6);
mv.visitJumpInsn(Opcodes.GOTO, tryLabel);
mv.visitVarInsn(Opcodes.ASTORE, 8);
mv.visitVarInsn(Opcodes.ALOAD, 0);
mv.visitVarInsn(Opcodes.ALOAD, 1);
mv.visitVarInsn(Opcodes.ALOAD, 2);
mv.visitVarInsn(Opcodes.ILOAD, 3);
mv.visitVarInsn(Opcodes.ILOAD, 4);
mv.visitVarInsn(Opcodes.ALOAD, 5);
mv.visitVarInsn(Opcodes.ALOAD, 8);
mv.visitVarInsn(Opcodes.ALOAD, 7);
mv.visitMethodInsn(Opcodes.INVOKESPECIAL, Type.getInternalName(ClassLoader.class),
"defineTransformedClass", "(Ljava/lang/String;[BIILjava/security/ProtectionDomain;Ljava/lang/ClassFormatError;Ljava/lang/String;)Ljava/lang/Class;");
mv.visitVarInsn(Opcodes.ASTORE, 6);
mv.visitLabel(tryLabel);
mv.visitVarInsn(Opcodes.ALOAD, 0);
mv.visitVarInsn(Opcodes.ALOAD, 6);
mv.visitVarInsn(Opcodes.ALOAD, 5);
mv.visitMethodInsn(Opcodes.INVOKESPECIAL, Type.getInternalName(ClassLoader.class),
"postDefineClass", "(Ljava/lang/Class;Ljava/security/ProtectionDomain;)V");
mv.visitVarInsn(Opcodes.ALOAD, 1);
mv.visitVarInsn(Opcodes.ALOAD, 6);
mv.visitMethodInsn(Opcodes.INVOKESTATIC, Type.getInternalName(Clazzs.class),
"setClazz", "(Ljava/lang/String;Ljava/lang/Class;)V");
mv.visitVarInsn(Opcodes.ALOAD, 6);
mv.visitInsn(Opcodes.ARETURN);
mv.visitLabel(jump);
}
};
}
return mv;
}
}
第二点的定时扫描文件，没什么难点。其中，根据class文件获取包名，前文已经分析过。具体参看github
说明下：hot2hot没有经过线上测试，使用请谨慎。