ClassLoader（二）- 加载过程

比眉伴天荒 2022-03-08 15:48 242阅读 0赞

本文源代码在[Github][]。

本文仅为个人笔记，不应作为权威参考。

[原文][Link 1]

在前一篇文章初步了解ClassLoader里提到了委托模型（又称双亲委派模型），解释了ClassLoader hierarchy（层级）处理类加载的过程。那么class文件是如何变成Class对象的呢？

## Class的加载过程 ##

Class加载分为这几步：

1.  创建和加载（Creation and Loading）
2.  链接（Linking）
    
    1.  验证（Verification）
    2.  准备（Preparation）
    3.  解析（Resolution），此步骤可选
3.  初始化（Initialization）

注: 前面说了数组类是虚拟机直接创建的，以上过程不适用于数组类。

## 创建和加载（Creation and Loading） ##

何时会触发一个类的加载？

[Java Language Specification - 12.1.1. Load the Class Test][]：

> The initial attempt to execute the method `main` of class `Test` discovers that the class `Test` is not loaded - that is, that the Java Virtual Machine does not currently contain a binary representation for this class. The Java Virtual Machine then uses a class loader to attempt to find such a binary representation.

也就是说，当要用到一个类，JVM发现还没有包含这个类的二进制形式（字节）时，就会使用ClassLoader尝试查找这个类的二进制形式。

我们知道ClassLoader委托模型，也就是说实际触发加载的ClassLoader和真正加载的ClassLoader可能不是同一个，JVM将它们称之为`initiating loader`和`defining loader`（[Java Virtual Machine Specification - 5.3. Creation and Loading][]）：

> A class loader `L` may create C by defining it directly or by delegating to another class loader. If `L` creates C directly, we say that `L` defines C or, equivalently, that `L` is the *defining loader* of C.
> 
> When one class loader delegates to another class loader, the loader that initiates the loading is not necessarily the same loader that completes the loading and defines the class. If `L` creates C, either by defining it directly or by delegation, we say that `L` initiates loading of C or, equivalently, that `L` is an *initiating loader* of C.

那么当A类使用B类的时候，B类使用的是哪个ClassLoader呢？

[Java Virtual Machine Specification - 5.3. Creation and Loading][]：

> The Java Virtual Machine uses one of three procedures to create class or interface C denoted by N:
> 
>  *  If `N` denotes a nonarray class or an interface, one of the two following methods is used to load and thereby create C:
>     
>      *  If D was defined by the bootstrap class loader, then the bootstrap class loader initiates loading of C (§5.3.1).
>      *  If D was defined by a user-defined class loader, then that same user-defined class loader initiates loading of C (§5.3.2).
>  *  Otherwise `N` denotes an array class. An array class is created directly by the Java Virtual Machine (§5.3.3), not by a class loader. However, the defining class loader of D is used in the process of creating array class C.
> 
> 注：上文的C和D都是类，N则是C的名字。

也就说如果D用到C，且C还没有被加载，且C不是数组，那么：

1.  如果D的defining loader是bootstrap class loader，那么C的initiating loader就是bootstrap class loader。
2.  如果D的defining loader是自定义的class loader X，那么C的initiating loader就是X。

再总结一下就是：如果D用到C，且C还没有被加载，且C不是数组，那么C的initiating loader就是D的defining loader。

用下面的代码观察一下：

// 把这个项目打包然后放到/tmp目录下
    public class CreationAndLoading {
      public static void main(String[] args) throws Exception {
        // ucl1的parent是bootstrap class loader
        URLClassLoader ucl1 = new NamedURLClassLoader("user-defined 1", new URL[] { new URL("file:///tmp/classloader.jar") }, null);
        // ucl1是ucl2的parent
        URLClassLoader ucl2 = new NamedURLClassLoader("user-defined 2", new URL[0], ucl1);
        Class<?> fooClass2 = ucl2.loadClass("me.chanjar.javarelearn.classloader.Foo");
        fooClass2.newInstance();
      }
    }
    
    public class Foo {
      public Foo() {
        System.out.println("Foo's classLoader: " + Foo.class.getClassLoader());
        System.out.println("Bar's classLoader: " + Bar.class.getClassLoader());
      }
    }
    
    public class NamedURLClassLoader extends URLClassLoader {
      private String name;
      public NamedURLClassLoader(String name, URL[] urls, ClassLoader parent) {
        super(urls, parent);
        this.name = name;
      }
      @Override
      protected Class<?> findClass(String name) throws ClassNotFoundException {
        System.out.println("ClassLoader: " + this.name + " findClass(" + name + ")");
        return super.findClass(name);
      }
      @Override
      public Class<?> loadClass(String name) throws ClassNotFoundException {
        System.out.println("ClassLoader: " + this.name + " loadClass(" + name + ")");
        return super.loadClass(name);
      }
      @Override
      public String toString() {
        return name;
      }
    }

运行结果是：

ClassLoader: user-defined 2 loadClass(me.chanjar.javarelearn.classloader.Foo)
    ClassLoader: user-defined 1 findClass(me.chanjar.javarelearn.classloader.Foo)
    ClassLoader: user-defined 1 loadClass(java.lang.Object)
    ClassLoader: user-defined 1 loadClass(java.lang.System)
    ClassLoader: user-defined 1 loadClass(java.lang.StringBuilder)
    ClassLoader: user-defined 1 loadClass(java.lang.Class)
    ClassLoader: user-defined 1 loadClass(java.io.PrintStream)
    Foo's classLoader: user-defined 1
    ClassLoader: user-defined 1 loadClass(me.chanjar.javarelearn.classloader.Bar)
    ClassLoader: user-defined 1 findClass(me.chanjar.javarelearn.classloader.Bar)
    Bar's classLoader: user-defined 1

可以注意到`Foo`的initiating loader是user-defined 2，但是defining loader是user-defined 1。而`Bar`的initiating loader与defining loader则直接是user-defined 1，绕过了user-defined 2。观察结果符合预期。

## 链接 ##

### 验证（Verification） ###

验证类的二进制形式在结构上是否正确。

### 准备（Preparation） ###

为类创建静态字段，并且为这些静态字段初始化默认值。

### 解析（Resolution） ###

JVM在运行时会为每个类维护一个run-time constant pool，run-time constant pool构建自类的二进制形式里的`constant_pool`表。run-time constant pool里的所有引用一开始都是符号引用（symbolic reference）（见[Java Virutal Machine Specification - 5.1. The Run-Time Constant Pool][]）。符号引用就是并非真正引用（即引用内存地址），只是指向了一个名字而已（就是字符串）。解析阶段做的事情就是将符号引用转变成实际引用）。

[Java Virutal Machine Specification - 5.4. Linking][]：

> This specification allows an implementation flexibility as to when linking activities (and, because of recursion, loading) take place, provided that all of the following properties are maintained:
> 
> 1.  A class or interface is completely loaded before it is linked.
> 2.  A class or interface is completely verified and prepared before it is initialized.

也就是说仅要求：

1.  一个类在被链接之前得是完全加载的。
2.  一个类在被初始化之前得是被完全验证和准备的。

所以对于解析的时机JVM Spec没有作出太多规定，只说了以下JVM指令在执行之前需要解析符号引用：\_anewarray\_, *checkcast\_, \_getfield\_, \_getstatic\_, \_instanceof\_, \_invokedynamic\_, \_invokeinterface\_, \_invokespecial\_, \_invokestatic\_, \_invokevirtual\_, \_ldc\_, \_ldc\_w\_, \_multianewarray\_, \_new\_, \_putfield* 和 *putstatic* 。

看不懂没关系，大致意思就是，用到字段、用到方法、用到静态方法、new类等时候需要解析符号引用。

## 初始化 ##

如果直接赋值的静态字段被 final 所修饰，并且它的类型是基本类型或字符串时，那么该字段便会被 Java 编译器标记成常量值（ConstantValue），其初始化直接由 Java 虚拟机完成。除此之外的直接赋值操作，以及所有静态代码块中的代码，则会被 Java 编译器置于同一方法中，并把它命为 `<clinit>`（**cl**ass **init**）。

JVM 规范枚举了下述类的初始化时机是：

1.  当虚拟机启动时，初始化用户指定的主类；
2.  new 某个类的时候
3.  调用某类的静态方法时
4.  访问某类的静态字段时
5.  子类初始化会触发父类初始化
6.  用反射API对某个类进行调用时
7.  一个接口定义了default方法（原文是non-abstract、non-static方法），某个实现了这个接口的类被初始化，那么这个接口也会被初始化
8.  初次调用 MethodHandle 实例时

注意：这里没有提到new 数组的情况，所以new 数组的时候不会初始化类。

同时类的初始化过程是线程安全的，下面是一个利用上述时机4和线程安全特性做的延迟加载的Singleton的例子：

public class Singleton {
      private Singleton() {}
      private static class LazyHolder {
        static final Singleton INSTANCE = new Singleton();
      }
      public static Singleton getInstance() {
        return LazyHolder.INSTANCE;
      }
    }

这种做法被称为[Initialization-on-demand holder idiom][]。

## 类加载常见异常 ##

### ClassNotFoundException ###

[Java Virutal Machine Specification - 5.3.1. Loading Using the Bootstrap Class Loader][]：

> If no purported representation of C is found, loading throws an instance of `ClassNotFoundException`.

[Java Virutal Machine Specification - 5.3.2. Loading Using a User-defined Class Loader][]：

> When the `loadClass` method of the class loader `L` is invoked with the name `N` of a class or interface C to be loaded, `L` must perform one of the following two operations in order to load C:
> 
> 1.  The class loader `L` can create an array of bytes representing C as the bytes of a ClassFile structure (§4.1); it then must invoke the method `defineClass` of class ClassLoader. Invoking defineClass causes the Java Virtual Machine to derive a class or interface denoted by `N` using `L` from the array of bytes using the algorithm found in §5.3.5.
> 2.  The class loader `L` can delegate the loading of C to some other class loader L'. This is accomplished by passing the argument `N` directly or indirectly to an invocation of a method on `L'` (typically the `loadClass` method). The result of the invocation is C.
> 
> In either (1) or (2), if the class loader `L` is unable to load a class or interface denoted by `N` for any reason, it must throw an instance of ClassNotFoundException.

所以，`ClassNotFoundException`发生在【加载阶段】：

1.  如果用的是bootstrap class loader，则当找不到其该类的二进制形式时抛出`ClassNotFoundException`
2.  如果用的是用户自定义class loader，不管是自己创建二进制（这里包括从文件读取或者内存中创建），还是代理给其他class loader，只要出现无法加载的情况，都要抛出`ClassNotFoundException`

### NoClassDefFoundError ###

[Java Virtual Machine Specification - 5.3. Creation and Loading][]

> If the Java Virtual Machine ever attempts to load a class C during verification (§5.4.1) or resolution (§5.4.3) (but not initialization (§5.5)), and the class loader that is used to initiate loading of C throws an instance of `ClassNotFoundException`, then the Java Virtual Machine must throw an instance of `NoClassDefFoundError` whose cause is the instance of `ClassNotFoundException`.
> 
> (A subtlety here is that recursive class loading to load superclasses is performed as part of resolution (§5.3.5, step 3). Therefore, a `ClassNotFoundException` that results from a class loader failing to load a superclass must be wrapped in a `NoClassDefFoundError`.)

[Java Virtual Machine Specification - 5.3.5. Deriving a Class from a class File Representation][]

> Otherwise, if the purported representation does not actually represent a class named `N`, loading throws an instance of `NoClassDefFoundError` or an instance of one of its subclasses.

[Java Virtual Machine Specification - 5.5. Initialization][Java Virutal Machine Specification - 5.4. Linking]

> If the Class object for C is in an erroneous state, then initialization is not possible. Release `LC` and throw a `NoClassDefFoundError`.

所以，`NoClassDefFoundError`发生在：

1.  【加载阶段】，因其他类的【验证】or【解析】触发对C类的【加载】，此时发生了`ClassNotFoundException`，那么就要抛出`NoClassDefFoundError`，cause 是`ClassNotFoundException`。
2.  【加载阶段】，在【解析】superclass的过程中发生的`ClassNotFoundException`也必须包在`NoClassDefFoundError`里。
3.  【加载阶段】，发现找到的二进制里的类名和要找的类名不一致时，抛出`NoClassDefFoundError`
4.  【初始化阶段】，如果C类的Class对象处于错误状态，那么抛出`NoClassDefFoundError`

## 追踪类的加载 ##

可以在JVM启动时添加`-verbose:class`来打印类加载过程。

## 参考资料 ##

*  [Java Language Specification - Chapter 12. Execution][]
 *  [Java Virtual Machine Specification - Chapter 5. Loading, Linking, and Initializing][Java Virtual Machine Specification - Chapter 5. Loading_ Linking_ and Initializing]
 *  [极客时间 - 深入拆解Java虚拟机 - 03 Java虚拟机是如何加载Java类的?][- _Java_ - 03 Java_Java]（专栏文章，需付费购买）
 *  [CS-Note 类加载机制][CS-Note]
 *  [深入理解JVM(八)——类加载的时机][JVM]
 *  [深入理解JVM(九)——类加载的过程][JVM 1]

## 广告 ##

![clipboard.png][]

[Github]: https://github.com/chanjarster/java-relearn/tree/master/classloader
[Link 1]: https://chanjarster.github.io/post/jvm/classloader/2-steps/
[Java Language Specification - 12.1.1. Load the Class Test]: https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.1.1
[Java Virtual Machine Specification - 5.3. Creation and Loading]: https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-5.html#jvms-5.3
[Java Virutal Machine Specification - 5.1. The Run-Time Constant Pool]: https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-5.html#jvms-5.1
[Java Virutal Machine Specification - 5.4. Linking]: https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-5.html#jvms-5.5
[Initialization-on-demand holder idiom]: https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom
[Java Virutal Machine Specification - 5.3.1. Loading Using the Bootstrap Class Loader]: https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-5.html#jvms-5.3.1
[Java Virutal Machine Specification - 5.3.2. Loading Using a User-defined Class Loader]: https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-5.html#jvms-5.3.2
[Java Virtual Machine Specification - 5.3.5. Deriving a Class from a class File Representation]: https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-5.html#jvms-5.3.5
[Java Language Specification - Chapter 12. Execution]: https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html
[Java Virtual Machine Specification - Chapter 5. Loading_ Linking_ and Initializing]: https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-5.html
[- _Java_ - 03 Java_Java]: https://time.geekbang.org/column/article/11523
[CS-Note]: https://cyc2018.github.io/CS-Notes/#/notes/Java%20%E8%99%9A%E6%8B%9F%E6%9C%BA?id=%E5%9B%9B%E3%80%81%E7%B1%BB%E5%8A%A0%E8%BD%BD%E6%9C%BA%E5%88%B6
[JVM]: https://blog.csdn.net/u010425776/article/details/51251430
[JVM 1]: https://blog.csdn.net/u010425776/article/details/51254858
[clipboard.png]: /images/20220308/5eba3c6f5a514924813af95a394be5b7.png