骚操作：不重启 JVM，如何替换掉已经加载的类？
小哈学Java
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2021-12-18 14:54
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本文来源：美团技术博客
Java对象行为 java.lang.instrument.Instrumentation 直接操作字节码 BTrace Arthas 三生万物
Java对象行为
publicclass Person{
privateint age;
private String name;
public void speak(String str) {
System.out.println(str);
}
public Person(int age, String name) {
this.age = age;
this.name = name;
}
}
Person personA = new Person(43, "lixunhuan");
personA.speak("我是李寻欢");
Person personB = new Person(23, "afei");
personB.speak("我是阿飞");
Method area is created on virtual machine startup, shared among all Java virtual machine threads and it is logically part of heap area. It stores per-class structures such as the run-time constant pool, field and method data, and the code for methods and constructors.
java.lang.instrument.Instrumentation。”java.lang.instrument.Instrumentation
This method is used to replace the definition of a class without reference to the existing class file bytes, as one might do when recompiling from source for fix-and-continue debugging. Where the existing class file bytes are to be transformed (for example in bytecode instrumentation) retransformClasses should be used.
The redefinition may change method bodies, the constant pool and attributes. The redefinition must not add, remove or rename fields or methods, change the signatures of methods, or change inheritance. These restrictions maybe be lifted in future versions. The class file bytes are not checked, verified and installed until after the transformations have been applied, if the resultant bytes are in error this method will throw an exception.
直接操作字节码
BTrace
在我们的工程中，谁来做这个寻找字节码，修改字节码，然后retransform的动作呢？我们并非先知，不可能知道未来有没有可能遇到文章开头的这种问题。考虑到性价比，我们也不可能在每个工程中都开发一段专门做这些修改字节码、重新加载字节码的代码。 如果JVM不在本地，在远程呢？ 如果连ASM都不会用呢？能不能更通用一些，更“傻瓜”一些。
A safe, dynamic tracing tool for the Java platform.
package com.sun.btrace.samples;
import com.sun.btrace.annotations.*;
import com.sun.btrace.AnyType;
importstatic com.sun.btrace.BTraceUtils.*;
/**
* This sample demonstrates regular expression
* probe matching and getting input arguments
* as an array - so that any overload variant
* can be traced in "one place". This example
* traces any "readXX" method on any class in
* java.io package. Probed class, method and arg
* array is printed in the action.
*/
@BTracepublicclass ArgArray {
@OnMethod(
clazz="/java\\.io\\..*/",
method="/read.*/"
)
public static void anyRead(@ProbeClassName String pcn, @ProbeMethodName String pmn, AnyType[] args) {
println(pcn);
println(pmn);
printArray(args);
}
}
package com.sun.btrace.samples;
import com.sun.btrace.annotations.*;
importstatic com.sun.btrace.BTraceUtils.*;
import com.sun.btrace.annotations.Export;
/**
* This sample creates a jvmstat counter and
* increments it everytime Thread.start() is
* called. This thread count may be accessed
* from outside the process. The @Export annotated
* fields are mapped to jvmstat counters. The counter
* name is "btrace." + + "." +
*/
@BTracepublicclass ThreadCounter {
// create a jvmstat counter using @Export
@Exportprivatestaticlong count;
@OnMethod(
clazz="java.lang.Thread",
method="start"
)
public static void onnewThread(@Self Thread t) {
// updating counter is easy. Just assign to
// the static field!
count++;
}
@OnTimer(2000)
public static void ontimer() {
// we can access counter as "count" as well
// as from jvmstat counter directly.
println(count);
// or equivalently ...
println(Counters.perfLong("btrace.com.sun.btrace.samples.ThreadCounter.count"));
}
}
BTrace脚本：利用BTrace定义的注解，我们可以很方便地根据需要进行脚本的开发。 Compiler：将BTrace脚本编译成BTrace class文件。 Client：将class文件发送到Agent。 Agent：基于Java的Attach API，Agent可以动态附着到一个运行的JVM上，然后开启一个BTrace Server，接收client发过来的BTrace脚本；解析脚本，然后根据脚本中的规则找到要修改的类；修改字节码后，调用Java Instrument的retransform接口，完成对对象行为的修改并使之生效。
不允许创建对象 不允许创建数组 不允许抛异常 不允许catch异常 不允许随意调用其他对象或者类的方法，只允许调用com.sun.btrace.BTraceUtils中提供的静态方法（一些数据处理和信息输出工具） 不允许改变类的属性 不允许有成员变量和方法，只允许存在static public void方法 不允许有内部类、嵌套类 不允许有同步方法和同步块 不允许有循环 不允许随意继承其他类（当然，java.lang.Object除外） 不允许实现接口 不允许使用assert 不允许使用Class对象
Arthas
三生万物
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