spring-instrument.tomcat解析之一：Tomcat默许类加载器的扩展

spring-instrument.tomcat解析之一：Tomcat默认类加载器的扩展
1、在开始之前，建议先阅读这篇文章http://xj84.iteye.com/blog/1221105（利用ClassFileTransformer实现aop）

2、TomcatInstrumentableClassLoader：Tomcat默认类加载器的扩展，添加了instrumentation

 

package org.springframework.instrument.classloading.tomcat;

import java.lang.instrument.ClassFileTransformer;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;

import org.apache.catalina.loader.ResourceEntry;
import org.apache.catalina.loader.WebappClassLoader;
import org.springframework.instrument.classloading.WeavingTransformer;

/**
 * [Extension of Tomcat's default class loader] 【which】 [adds] [instrumentation]
 * [to loaded] [classes] (without the need to use a VM-wide agent).
 * Tomcat默认类加载器的扩展，添加了instrumentation，在不使用VM级别的代理情况下就加载类
 *可以在tomcat的server.xml问中配置，代替默认的类加载器
 * <p>To be registered using a 
 * <code><a href="http://tomcat.apache.org/tomcat-5.5-doc/config/loader.html">Loader</a></code> tag 
 * in Tomcat's <code><a href="http://tomcat.apache.org/tomcat-5.5-doc/config/context.html">Context</a></code>
 * definition in the <code>server.xml</code> file, with the Spring-provided
 * "spring-tomcat-weaver.jar" file deployed into Tomcat's "server/lib" (for Tomcat 5.x) or "lib" (for Tomcat 6.x) directory.
 * The required configuration tag looks as follows:
 *
 * <pre class="code">&lt;Loader loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader"/&gt;</pre>
 *
 * <p>Typically used in combination with a
 * {@link org.springframework.instrument.classloading.ReflectiveLoadTimeWeaver}
 * defined in the Spring application context. The <code>addTransformer</code> and
 * <code>getThrowawayClassLoader</code> methods mirror the corresponding methods
 * in the LoadTimeWeaver interface, as expected by ReflectiveLoadTimeWeaver.
 *
 * <p>See the PetClinic sample application for a full example of this
 * ClassLoader in action.
 *
 * <p><b>NOTE:</b> Requires Apache Tomcat version 5.0 or higher.
 *
 * @author Costin Leau
 * @author Juergen Hoeller
 * @since 2.0
 * @see #addTransformer
 * @see #getThrowawayClassLoader
 * @see org.springframework.instrument.classloading.ReflectiveLoadTimeWeaver
 */
public class TomcatInstrumentableClassLoader extends WebappClassLoader {

	private static final String CLASS_SUFFIX = ".class";
	/** Use an internal WeavingTransformer */
	private final WeavingTransformer weavingTransformer;//转换织入者


	/**
	 * Create a new <code>TomcatInstrumentableClassLoader</code> using the
	 * current context class loader.
	 * @see #TomcatInstrumentableClassLoader(ClassLoader)
	 */
	public TomcatInstrumentableClassLoader() {
		super();
		this.weavingTransformer = new WeavingTransformer(this);
	}

	/**
	 * Create a new <code>TomcatInstrumentableClassLoader</code> with the
	 * supplied class loader as parent.
	 * @param parent the parent {@link ClassLoader} to be used
	 */
	public TomcatInstrumentableClassLoader(ClassLoader parent) {
		super(parent);
		this.weavingTransformer = new WeavingTransformer(this);
	}


	/**
	 * Delegate for LoadTimeWeaver's <code>addTransformer</code> method.
	 * Typically called through ReflectiveLoadTimeWeaver.
	 * @see org.springframework.instrument.classloading.LoadTimeWeaver#addTransformer
	 * @see org.springframework.instrument.classloading.ReflectiveLoadTimeWeaver
	 */
	public void addTransformer(ClassFileTransformer transformer) {
		this.weavingTransformer.addTransformer(transformer);
	}

	/**
	 * Delegate for LoadTimeWeaver's <code>getThrowawayClassLoader</code> method.
	 * Typically called through ReflectiveLoadTimeWeaver.
	 * @see org.springframework.instrument.classloading.LoadTimeWeaver#getThrowawayClassLoader
	 * @see org.springframework.instrument.classloading.ReflectiveLoadTimeWeaver
	 */
	public ClassLoader getThrowawayClassLoader() {
		WebappClassLoader tempLoader = new WebappClassLoader();
		// Use reflection to copy all the fields since most of them are private
		// on pre-5.5 Tomcat.
		shallowCopyFieldState(this, tempLoader);
		return tempLoader;
	}


	@Override
	protected ResourceEntry findResourceInternal(String name, String path) {
		//在加载查询资源时实现类转换
		ResourceEntry entry = super.findResourceInternal(name, path);
		// Postpone String parsing as much as possible (it is slow).
		if (entry != null && entry.binaryContent != null && path.endsWith(CLASS_SUFFIX)) {
			String className = (name.endsWith(CLASS_SUFFIX) ? name.substring(0, name.length() - CLASS_SUFFIX.length())
					: name);
			byte[] transformed = this.weavingTransformer.transformIfNecessary(className, entry.binaryContent);
			entry.binaryContent = transformed;
		}
		return entry;
	}

	@Override
	public String toString() {
		StringBuilder sb = new StringBuilder(getClass().getName());
		sb.append("\r\n");
		sb.append(super.toString());
		return sb.toString();
	}


	// The code below is originally taken from ReflectionUtils and optimized for
	// local usage. There is no dependency on ReflectionUtils to keep this class
	// self-contained (since it gets deployed into Tomcat's server class loader).

	/**
	 * [Given] [the source object] and [the destination], (which must be the same class
	 * or a subclass), [copy][all fields, including inherited fields]. [Designed] [to
	 * work on] [objects] (with public no-arg constructors).
	 * 给定源对象和目标对象，二者必须是同类型或子类型
	 * 拷贝包括继承属性在内的所有属性值
	 * 该方法设计用于不带参数的公有构造函数对象
	 * @throws IllegalArgumentException if arguments are incompatible or either
	 * is <code>null</code>
	 */
	private static void shallowCopyFieldState(final Object src, final Object dest) throws IllegalArgumentException {
		if (src == null) {
			throw new IllegalArgumentException("Source for field copy cannot be null");
		}
		if (dest == null) {
			throw new IllegalArgumentException("Destination for field copy cannot be null");
		}
		Class targetClass = findCommonAncestor(src.getClass(), dest.getClass());

		// Keep backing up the inheritance hierarchy.
		do {
			// Copy each field declared on this class unless it's static or
			// file.
			Field[] fields = targetClass.getDeclaredFields();
			for (int i = 0; i < fields.length; i++) {
				Field field = fields[i];
				// Skip static and final fields (the old FieldFilter)
				// do not copy resourceEntries - it's a cache that holds class entries.
				if (!(Modifier.isStatic(field.getModifiers()) || Modifier.isFinal(field.getModifiers()) ||
						field.getName().equals("resourceEntries"))) {
					try {
						// copy the field (the old FieldCallback)
						field.setAccessible(true);
						Object srcValue = field.get(src);
						field.set(dest, srcValue);
					}
					catch (IllegalAccessException ex) {
						throw new IllegalStateException(
								"Shouldn't be illegal to access field '" + fields[i].getName() + "': " + ex);
					}
				}
			}
			targetClass = targetClass.getSuperclass();//遍历所有父类
		}
		while (targetClass != null && targetClass != Object.class);
	}

	private static Class findCommonAncestor(Class one, Class two) throws IllegalArgumentException {
		Class ancestor = one;
		while (ancestor != Object.class || ancestor != null) {
			if (ancestor.isAssignableFrom(two)) {
				return ancestor;
			}
			ancestor = ancestor.getSuperclass();
		}
		// try the other class hierarchy
		ancestor = two;
		while (ancestor != Object.class || ancestor != null) {
			if (ancestor.isAssignableFrom(one)) {
				return ancestor;
			}
			ancestor = ancestor.getSuperclass();
		}
		return null;
	}

}

3、WeavingTransformer：基于ClassFileTransformer的编织者，允许一系列转换者（transformer）织入到类字节码中。
把目标比作衣服，transformer就是衣服上的装饰，而WeavingTransformer就是缝纫机

 

package org.springframework.instrument.classloading;

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;
import java.util.ArrayList;
import java.util.List;

/**
 * [ClassFileTransformer-based weaver], [allowing for] [a list of transformers] [to be
 * applied on] [a class byte array]. Normally used inside class loaders.
 *基于ClassFileTransformer的编织者，允许一系列转换者（transformer）织入到类字节码中。
 *通常用在类加载器内
 * <p>Note: [This class] is [deliberately implemented] [for minimal external dependencies],
 * [since it [is included] in weaver jars (to be deployed into application servers)].
 *此类尽量少的使用外部引用，因为它被包含在织入者jar包内
 * @author Rod Johnson
 * @author Costin Leau
 * @author Juergen Hoeller
 * @since 2.0
 */
public class WeavingTransformer {

	private final ClassLoader classLoader;

	private final List<ClassFileTransformer> transformers = new ArrayList<ClassFileTransformer>();


	/**
	 * Create a new WeavingTransformer for the given class loader.
	 * @param classLoader the ClassLoader to build a transformer for
	 */
	public WeavingTransformer(ClassLoader classLoader) {
		if (classLoader == null) {
			throw new IllegalArgumentException("ClassLoader must not be null");
		}
		this.classLoader = classLoader;
	}


	/**
	 * Add a class file transformer to be applied by this weaver.
	 * 添加可以织入的转换者
	 * @param transformer the class file transformer to register
	 */
	public void addTransformer(ClassFileTransformer transformer) {
		if (transformer == null) {
			throw new IllegalArgumentException("Transformer must not be null");
		}
		this.transformers.add(transformer);
	}


	/**
	 * Apply transformation on a given class byte definition.
	 * The method will always return a non-null byte array (if no transformation has taken place
	 * the array content will be identical to the original one).
	 * @param className the full qualified name of the class in dot format (i.e. some.package.SomeClass)
	 * @param bytes class byte definition
	 * @return (possibly transformed) class byte definition
	 */
	public byte[] transformIfNecessary(String className, byte[] bytes) {
		String internalName = className.replace(".", "/");
		return transformIfNecessary(className, internalName, bytes, null);
	}

	/**
	 * Apply transformation on a given class byte definition.
	 * 在一个给定的class字节码中实施转换
	 * The method will always return a non-null byte array (if no transformation has taken place
	 * the array content will be identical to the original one).
	 * 该函数返回非null值，如果没有转换则返回本身
	 * @param className the full qualified name of the class in dot format (i.e. some.package.SomeClass)
	 * @param internalName class name internal name in / format (i.e. some/package/SomeClass)
	 * @param bytes class byte definition
	 * @param pd protection domain to be used (can be null)
	 * @return (possibly transformed) class byte definition
	 */
	public byte[] transformIfNecessary(String className, String internalName, byte[] bytes, ProtectionDomain pd) {
		byte[] result = bytes;
		for (ClassFileTransformer cft : this.transformers) {//遍历所有转换者，依次转换
			try {
				//转换的内容在ClassFileTransformer的transform内执行
				byte[] transformed = cft.transform(this.classLoader, internalName, null, pd, result);
				if (transformed != null) {
					result = transformed;
				}
			}
			catch (IllegalClassFormatException ex) {
				throw new IllegalStateException("Class file transformation failed", ex);
			}
		}
		return result;
	}

}