0.1 、Spring 源码学习-JVM类加载器-双亲委派模型-Spring 中类加载机制

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JVM 类加载机制

/**
* 获取类加载器的测试方法
*/
@Test
public void test2() {
	//获取Test类的类加载器 sun.misc.Launcher$AppClassLoader@4dc63996
	ClassLoader c  = Test.class.getClassLoader();  
       System.out.println(c); 
       //获取c这个类加载器的父类加载器 sun.misc.Launcher$ExtClassLoader@28a418fc
       ClassLoader c1 = c.getParent(); 
       System.out.println(c1); 
       // getClassLoader() returning null indicates the bootstrap ClassLoader
       //获取c1这个类加载器的父类加载器 ,null,因为Bootstrp loader是C++编写的,依java的观点来看，逻辑上并不存在Bootstrap Loader的类实体
       //根装载器:Bootstrp loader
       //用C++语言写的，它是在Java虚拟机启动后初始化的，主要负责加载%JAVA_HOME%/jre/lib,
       //* -Xbootclasspath参数指定的路径以及%JAVA_HOME%/jre/classes中的类。
       ClassLoader c2 = c1.getParent();
       System.out.println(c2);
       
       //获取系统默认的ClassLoader sun.misc.Launcher$AppClassLoader@4dc63996
       ClassLoader c3=ClassLoader.getSystemClassLoader();
       System.out.println(c3);
       //获取创建当前线程的类加载器 
       ClassLoader c4= Thread.currentThread().getContextClassLoader();
       System.out.println(c4);
}

双亲委派模型

简单说就是,先由自己的父类加载需要的类,一直向上传递,如果没有再由自己加载
类加载器可以重写,但是核心类是不允许使用自己的类加载器加载的,比如 jdk 核心包 java.lang.String ,只允许 bootstrap ClassLoader 进行加载

Spring 类加载机制

入口 AbstractApplicationContext.prepareBeanFactory

beanFactory.setBeanClassLoader(getClassLoader());

DefaultResourceLoader.getClassLoader()

org.springframework.core.io.DefaultResourceLoader

/**
* Return the ClassLoader to load class path resources with.
 * <p>Will get passed to ClassPathResource's constructor for all
 * ClassPathResource objects created by this resource loader.
 * @see ClassPathResource
 */
@Override
public ClassLoader getClassLoader() {
	//如果指定了类加载器就返回指定的类加载器,否则获取线程类加载器->不存在就获取ClassUtils类加载器-> 不存在就获取系统默认类加载器
	return (this.classLoader != null ? this.classLoader : ClassUtils.getDefaultClassLoader());
}

ClassUtils.getDefaultClassLoader()

org.springframework.util.ClassUtils.getDefaultClassLoader()
看中文注释

/**
* Return the default ClassLoader to use: typically the thread context
 * ClassLoader, if available; the ClassLoader that loaded the ClassUtils
 * class will be used as fallback.
 * <p>Call this method if you intend to use the thread context ClassLoader
 * in a scenario where you clearly prefer a non-null ClassLoader reference:
 * for example, for class path resource loading (but not necessarily for
 * {@code Class.forName}, which accepts a {@code null} ClassLoader
 * reference as well).
 * @return the default ClassLoader (only {@code null} if even the system
 * ClassLoader isn't accessible)
 * @see Thread#getContextClassLoader()
 * @see ClassLoader#getSystemClassLoader()
 */
public static ClassLoader getDefaultClassLoader() {
	ClassLoader cl = null;
	try {
		//获取当前线程类加载器
		cl = Thread.currentThread().getContextClassLoader();
	}
	catch (Throwable ex) {
		// Cannot access thread context ClassLoader - falling back...
	}
	//如果线程类加载器为null
	if (cl == null) {
		// No thread context class loader -> use class loader of this class.
		//获取当前类的类加载器
		cl = ClassUtils.class.getClassLoader();
		if (cl == null) {
			// getClassLoader() returning null indicates the bootstrap ClassLoader
			//getClassLoader() 为 null 说明是 bootstrap ClassLoader 类加载器,该加载器为 C++ 编写,故从Java 角度看是 null
			try {
				//获取当前系统的类加载器
				cl = ClassLoader.getSystemClassLoader();
			}
			catch (Throwable ex) {
				// Cannot access system ClassLoader - oh well, maybe the caller can live with null...
			}
		}
	}
	return cl;
}