Java LinkageError Loader Constraint 分析

前言

作为 java 程序员, 我们经常遇到该错误, 特别是在传统的 web 容器里, 例如 tomcat, jetty;

例如 stackoverflow 上的经典错误;

StandardClassLoader 尝试加载 ExpressionFactory, 但 ExpressionFactory 已经被 JasperLoader 加载过了;

https://stackoverflow.com/questions/8487048/java-lang-linkageerror-javax-servlet-jsp-jspapplicationcontext-getexpressionfac

通常解决办法就是将 JasperLoader classpath 里的 javax/el/ExpressionFactory 删掉, 或者设置为 provide, 不要让 JasperLoader 去加载这个类.

exception

javax.servlet.ServletException: java.lang.LinkageError: loader constraint violation: when resolving interface method "javax.servlet.jsp.JspApplicationContext.getExpressionFactory()Ljavax/el/ExpressionFactory;
" the class loader (instance of org/apache/jasper/servlet/JasperLoader) of the current class, org/apache/jsp/index_jsp, and the class loader (instance of org/apache/catalina/loader/StandardClassLoader) for resolved class, javax/servlet/jsp/JspApplicationContext, 
have different Class objects for the type javax/el/ExpressionFactory used in the signature
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:343)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)

root cause

java.lang.LinkageError: loader constraint violation: when resolving interface method "javax.servlet.jsp.JspApplicationContext.getExpressionFactory()Ljavax/el/ExpressionFactory;" the class loader (instance of org/apache/jasper/servlet/JasperLoader) of the current class, org/apache/jsp/index_jsp, and the class loader (instance of org/apache/catalina/loader/StandardClassLoader) for resolved class, javax/servlet/jsp/JspApplicationContext, have different Class objects for the type javax/el/ExpressionFactory used in the signature
org.apache.jsp.index_jsp._jspInit(index_jsp.java:31)
org.apache.jasper.runtime.HttpJspBase.init(HttpJspBase.java:49)
org.apache.jasper.servlet.JspServletWrapper.getServlet(JspServletWrapper.java:180)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:369)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)

本文重点是如何复现 LinkageError 和分析 LinkageError 出现的原因;

分析

1. 这是个什么错误?
2. 为什么会出现这个错误?

GPT 回答: java.lang.LinkageError: loader constraint violation 是 Java 抛出的一个错误，表示类加载器之间存在一个不允许的情况。具体来说，当两个不同的类加载器试图加载相同完全限定名的类，但为这个名字指向了不同版本的类文件时，这种冲突就会发生。

再简单一点说:

假设有C1类和C2类都依赖C0，C1和C2分别用不同的2个类加载器加载，
而这两个类加载器都能在自己的类加载路径中加载到C0，
这个时候如果在C1中调用C2的某个方法（注：这个方法的签名中依赖了C0）
就会出现LinkageError错误。

注意:

1. 这2个类加载器(org.example.SFClassloader)是 self-first 的, 即违反了双亲委派的类加载器.
2. 这2个类加载器是父子结构
3. c1 和 c2 这 2 个目录都包含 C0 这个类.

结构:

public class C0 {
    public C0() {
    }
}

public class C1 {
    public C1() {
    }

    public C0 get() {
        return new C0();
    }
}

public class C2 {
    C0 c0 = new C0();

    public C2() {
    }

    public C0 get() {
        return (new C1()).get();
    }
}

执行下面的代码即可复现:

URL u = new File("c1/").toURI().toURL();
C1SFClassloader c1SFClassloader = new C1SFClassloader(new URL[]{u}, Thread.currentThread().getContextClassLoader());

URL u2 = new File("c2/").toURI().toURL();
// 父子结构
C2SFClassloader c2SFClassloader = new C2SFClassloader(new URL[]{u2}, c1SFClassloader);
// 加载 c2, c2 cinit 会主动加载 c0
Class<?> aClass = c2SFClassloader.loadClass("org.example.C2");
Object o = aClass.newInstance();
Method get = aClass.getMethod("get");

// 调用 get, c2 会访问 c1, c1 则会再次加载 c0, 触发 error 
Object invoke = get.invoke(o);

项目地址: https://github.com/stateIs0/linkageError-test

以下是 hotspot 虚拟机里关于这个错误的相关代码

// Constraints on class loaders. The details of the algorithm can be
// found in the OOPSLA'98 paper "Dynamic Class Loading in the Java
// Virtual Machine" by Sheng Liang and Gilad Bracha.  The basic idea is
// that the system dictionary needs to maintain a set of contraints that
// must be satisfied by all classes in the dictionary.
// if defining is true, then LinkageError if already in systemDictionary
// if initiating loader, then ok if InstanceKlass matches existing entry

// 对类加载器的约束。算法的详细信息可以在 OOPSLA'98 的论文 "Dynamic Class Loading in the Java
// Virtual Machine" 中找到，该论文由 Sheng Liang 和 Gilad Bracha 编写。基本的思想是
// 系统字典需要维护一个约束集，所有在字典中的类都必须满足这些约束。
// 如果 defining 为 true，则在 systemDictionary 中已存在时会抛出 LinkageError
// 如果是初始化加载器，则当 InstanceKlass 与现有条目匹配时是可以的。

void SystemDictionary::check_constraints(int d_index, unsigned int d_hash,
                                         instanceKlassHandle k,
                                         Handle class_loader, bool defining,
                                         TRAPS) {
  const char *linkage_error = NULL;
  {
    Symbol*  name  = k->name();
    ClassLoaderData *loader_data = class_loader_data(class_loader);

    MutexLocker mu(SystemDictionary_lock, THREAD);

    Klass* check = find_class(d_index, d_hash, name, loader_data);
    if (check != (Klass*)NULL) {
      // if different InstanceKlass - duplicate class definition,
      // else - ok, class loaded by a different thread in parallel,
      // we should only have found it if it was done loading and ok to use
      // system dictionary only holds instance classes, placeholders
      // also holds array classes
      
    // 如果是不同的 InstanceKlass - 表示类的重复定义，
    // 否则 - 没问题，类是由不同的线程并行加载的，
    // 我们只应该找到它，如果它已经完成加载并且可以使用。
    // 系统字典只存储实例类，
    // 占位符还存储数组类。

      assert(check->oop_is_instance(), "noninstance in systemdictionary");
      if ((defining == true) || (k() != check)) {
        linkage_error = "loader (instance of  %s): attempted  duplicate class "
          "definition for name: \"%s\"";
      } else {
        return;
      }
    }

#ifdef ASSERT
    Symbol* ph_check = find_placeholder(name, loader_data);
    assert(ph_check == NULL || ph_check == name, "invalid symbol");
#endif

    if (linkage_error == NULL) {
      if (constraints()->check_or_update(k, class_loader, name) == false) {
        linkage_error = "loader constraint violation: loader (instance of %s)"
          " previously initiated loading for a different type with name \"%s\"";
      }
    }
  }

  // Throw error now if needed (cannot throw while holding
  // SystemDictionary_lock because of rank ordering)

  if (linkage_error) {
    ResourceMark rm(THREAD);
    const char* class_loader_name = loader_name(class_loader());
    char* type_name = k->name()->as_C_string();
    size_t buflen = strlen(linkage_error) + strlen(class_loader_name) +
      strlen(type_name);
    char* buf = NEW_RESOURCE_ARRAY_IN_THREAD(THREAD, char, buflen);
    jio_snprintf(buf, buflen, linkage_error, class_loader_name, type_name);
    THROW_MSG(vmSymbols::java_lang_LinkageError(), buf);
  }
}

这段代码在Java虚拟机中确保了类加载约束的满足，并确保不会有重复的类定义。如果违反了这些规则，它会抛出一个LinkageError异常。

注释里提到的论文: https://dl.acm.org/doi/10.1145/286942.286945

maven 模块依赖复现

Maven 依赖

日常情况下,我们经常在 maven 模块里出现该错误;此处,我们使用 maven 模块依赖, 进行复现.

类 依赖

整体 依赖

运行 org.example.Main#main

son org.example.HelloImplSon start, 
son org.example.HelloInterface start, 
son java.lang.Object start, 
parent java.lang.Object start, 
son java.lang.Throwable start, 
objc[79363]: Class JavaLaunchHelper is implemented in both /Library/Java/JavaVirtualMachines/jdk1.8.0_72.jdk/Contents/Home/bin/java (0x10235b4c0) and /Library/Java/JavaVirtualMachines/jdk1.8.0_72.jdk/Contents/Home/jre/lib/libinstrument.dylib (0x10aa884e0). One of the two will be used. Which one is undefined.
java.lang.Object
java.lang.Object
son org.example.HelloInterface end, org.example.SonClassLoader@266474c2
son org.example.HelloImplSon end, org.example.SonClassLoader@266474c2
java.lang.Throwable
java.lang.Throwable
parent java.lang.Throwable start, 
son java.lang.Error start, 
java.lang.Error
parent java.lang.Error start, 
java.lang.Error
son org.example.HelloModel start, 
son org.example.HelloModel end, org.example.SonClassLoader@266474c2
son org.example.Delegate start, 
org.example.Delegate
parent org.example.Delegate start, 
parent java.lang.Object start, 
parent org.example.HelloInterface start, 
java.lang.Object
parent org.example.Delegate end, org.example.ParentClassLoader@7440e464
java.lang.LinkageError: loader constraint violation: loader (instance of org/example/ParentClassLoader) previously initiated loading for a different type with name "org/example/HelloInterface"
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:760)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
	at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
	at org.example.MyClassLoader.loadClass(MyClassLoader.java:76)
	at org.example.HelloImplSon.hello(HelloImplSon.java:28)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.example.Main.main(Main.java:18)

这个项目也是使用 Self first 模式, 和上面的 c0, c1, c2 类似;
项目地址: https://github.com/stateIs0/linkageError-test

总结

要复现 LinkageError 核心条件:

1. 两个 CL 是 self-first 模式
2. 两个类加载器是父子类加载器(相对于平行类加载器)
3. 父子 CL classpath都有同名类, 且有交叉访问(例如 C2 里访问 C1, C2 和 C1 都有 C0).

参考

• https://blog.csdn.net/helowken2/article/details/87927831
• https://docs.jboss.org/jbossas/docs/Server_Configuration_Guide/4/html/Class_Loading_and_Types_in_Java-LinkageErrors___Making_Sure_You_Are_Who_You_Say_You_Are.html
• http://frankkieviet.blogspot.com/2009/03/javalanglinkageerror-loader-constraint.html