我们知道，在 JVM 中，一个类加载的过程大致分为加载、链接（验证、准备、解析）、初始化5个阶段。而我们通常提到类的加载，
就是指利用类加载器（ClassLoader）通过类的全限定名来获取定义此类的二进制字节码流，进而构造出类的定义
我们先来卡一个普通的类加载顺序
从上图可知，通常的类加载都是委托给最顶成的启动类进行加载，flink同样提供了这样的加载器ParentFirstClassLoader
代码语言：javascript复制/**
* Regular URLClassLoader that first loads from the parent and only after that from the URLs.
*/
public static class ParentFirstClassLoader extends FlinkUserCodeClassLoader {
ParentFirstClassLoader(
URL[] urls, ClassLoader parent, Consumer<Throwable> classLoadingExceptionHandler) {
super(urls, parent, classLoadingExceptionHandler);
}
static {
ClassLoader.registerAsParallelCapable();
}
}不过flink作为一个分布式的计算引擎，经常会有一些第三方的jar需要被加载，全部委托给系统类加载不现实，如果仍然用双亲委派模型，就会因为 Flink 框架指定版本的类先加载，而出现莫名其妙的兼容性问题，如 NoSuchMethodError、IllegalAccessError 等，Flink 实现了 ChildFirstClassLoader 类加载器并作为默认策略。它打破了双亲委派模型，使得用户代码的类先加载，官方文档中将这个操作称为"Inverted Class Loading"
代码语言：javascript复制/**
* A variant of the URLClassLoader that first loads from the URLs and only after that from the
* parent.
*
* <p>{@link #getResourceAsStream(String)} uses {@link #getResource(String)} internally so we don't
* override that.
*/
public final class ChildFirstClassLoader extends FlinkUserCodeClassLoader {
/**
* The classes that should always go through the parent ClassLoader. This is relevant for Flink
* classes, for example, to avoid loading Flink classes that cross the user-code/system-code
* barrier in the user-code ClassLoader.
*/
private final String[] alwaysParentFirstPatterns;
public ChildFirstClassLoader(
URL[] urls,
ClassLoader parent,
String[] alwaysParentFirstPatterns,
Consumer<Throwable> classLoadingExceptionHandler) {
super(urls, parent, classLoadingExceptionHandler);
this.alwaysParentFirstPatterns = alwaysParentFirstPatterns;
}
@Override
protected Class<?> loadClassWithoutExceptionHandling(String name, boolean resolve)
throws ClassNotFoundException {
// First, check if the class has already been loaded
Class<?> c = findLoadedClass(name);
if (c == null) {
// check whether the class should go parent-first
for (String alwaysParentFirstPattern : alwaysParentFirstPatterns) {
if (name.startsWith(alwaysParentFirstPattern)) {
return super.loadClassWithoutExceptionHandling(name, resolve);
}
}
try {
// check the URLs
c = findClass(name);
} catch (ClassNotFoundException e) {
// let URLClassLoader do it, which will eventually call the parent
c = super.loadClassWithoutExceptionHandling(name, resolve);
}
} else if (resolve) {
resolveClass(c);
}
return c;
}
@Override
public URL getResource(String name) {
// first, try and find it via the URLClassloader
URL urlClassLoaderResource = findResource(name);
if (urlClassLoaderResource != null) {
return urlClassLoaderResource;
}
// delegate to super
return super.getResource(name);
}
@Override
public Enumeration<URL> getResources(String name) throws IOException {
// first get resources from URLClassloader
Enumeration<URL> urlClassLoaderResources = findResources(name);
final List<URL> result = new ArrayList<>();
while (urlClassLoaderResources.hasMoreElements()) {
result.add(urlClassLoaderResources.nextElement());
}
// get parent urls
Enumeration<URL> parentResources = getParent().getResources(name);
while (parentResources.hasMoreElements()) {
result.add(parentResources.nextElement());
}
return new Enumeration<URL>() {
Iterator<URL> iter = result.iterator();
public boolean hasMoreElements() {
return iter.hasNext();
}
public URL nextElement() {
return iter.next();
}
};
}
static {
ClassLoader.registerAsParallelCapable();
}
}下面在从类的继承图和调用的入口来分析一下
继承图：
调用图：
默认是ChildFirstClassLoader
下面看一下配置参数：
配置项 | 默认值 | 描述 |
|---|---|---|
classloader.resolve-order | child-first | 类加载顺序。child-first优先从Flink任务(jar包)中加载类，parent-first优先从Flink集群加载类。 |
classloader.parent-first-patterns.default | java.;scala.; org.apache.flink.; com.esotericsoftware.kryo;org.apache.hadoop.; javax.annotation.; org.slf4j;org.apache.log4j; org.apache.logging; org.apache.commons.logging;ch.qos.logback;org.xml; javax.xml; org.apache.xerces;org.w3c | 优先从Flink集群加载的类，以分号分隔的类前缀匹配；alwaysParentFirstPatterns 集合中的这些类都是 Java、Flink 等组件的基础，不能被用户代码冲掉 |
classloader.parent-first-patterns.additional | 用户如果有其他类以 child-first 模式会发生冲突，而希望以双亲委派模型来加载的话，可以额外指定（分号分隔） |
