介绍

ClassLoader 顾名思义就是类加载器 ClassLoader 是一个抽象类 没有父类

作用

1.负责将 Class 加载到 JVM 中

2.审查每个类由谁加载（父优先的等级加载机制）

3.将 Class 字节码重新解析成 JVM 统一要求的对象格式

常量&变量

 	//注册本地方法private static native void registerNatives();static {registerNatives();}// The parent class loader for delegation// Note: VM hardcoded the offset of this field, thus all new fields// must be added *after* it.//父类加载器，需要与BuiltinClassLoader中的parent区分private final ClassLoader parent;// Maps class name to the corresponding lock object when the current// class loader is parallel capable.// Note: VM also uses this field to decide if the current class loader// is parallel capable and the appropriate lock object for class loading.//当前类加载器具有并行功能时，将其下的类名映射到锁对象private final ConcurrentHashMap<String, Object> parallelLockMap;// Hashtable that maps packages to certs// 将包名映射到身份证书private final Map <String, Certificate[]> package2certs;// Shared among all packages with unsigned classesprivate static final Certificate[] nocerts = new Certificate[0];// The classes loaded by this class loader. The only purpose of this table// is to keep the classes from being GC'ed until the loader is GC'ed.// 记录当前类加载器加载的类private final Vector<Class<?>> classes = new Vector<>();// The "default" domain. Set as the default ProtectionDomain on newly// created classes.private final ProtectionDomain defaultDomain =new ProtectionDomain(new CodeSource(null, (Certificate[]) null),null, this, null);// The packages defined in this class loader.  Each package name is mapped// to its corresponding Package object.// @GuardedBy("itself")// 记录当前类加载器定义的包private final HashMap<String, Package> packages = new HashMap<>();// The class loader for the system// @GuardedBy("ClassLoader.class")//system class loader，可能是内置的AppClassLoader(默认)，也可能是自定义的类加载器private static ClassLoader scl;// Set to true once the system class loader has been set// @GuardedBy("ClassLoader.class")private static boolean sclSet;// All native library names we've loaded.private static Vector<String> loadedLibraryNames = new Vector<>();// Native libraries belonging to system classes.private static Vector<NativeLibrary> systemNativeLibraries= new Vector<>();// Native libraries associated with the class loader.private Vector<NativeLibrary> nativeLibraries = new Vector<>();// native libraries being loaded/unloaded.private static Stack<NativeLibrary> nativeLibraryContext = new Stack<>();// The paths searched for librariesprivate static String usr_paths[];private static String sys_paths[];final Object assertionLock;// The default toggle for assertion checking.// @GuardedBy("assertionLock")private boolean defaultAssertionStatus = false;// Maps String packageName to Boolean package default assertion status Note// that the default package is placed under a null map key.  If this field// is null then we are delegating assertion status queries to the VM, i.e.,// none of this ClassLoader's assertion status modification methods have// been invoked.// @GuardedBy("assertionLock")private Map<String, Boolean> packageAssertionStatus = null;// Maps String fullyQualifiedClassName to Boolean assertionStatus If this// field is null then we are delegating assertion status queries to the VM,// i.e., none of this ClassLoader's assertion status modification methods// have been invoked.// @GuardedBy("assertionLock")Map<String, Boolean> classAssertionStatus = null;

构造方法

 private ClassLoader(Void unused, ClassLoader parent) {this.parent = parent;//类加载器是否可并行if (ParallelLoaders.isRegistered(this.getClass())) {parallelLockMap = new ConcurrentHashMap<>();package2certs = new ConcurrentHashMap<>();assertionLock = new Object();} else {// no finer-grained lock; lock on the classloader instanceparallelLockMap = null;package2certs = new Hashtable<>();assertionLock = this;}}/*** Creates a new class loader using the specified parent class loader for* delegation.** <p> If there is a security manager, its {@link* SecurityManager#checkCreateClassLoader()* <tt>checkCreateClassLoader</tt>} method is invoked.  This may result in* a security exception.  </p>** @param  parent*         The parent class loader** @throws  SecurityException*          If a security manager exists and its*          <tt>checkCreateClassLoader</tt> method doesn't allow creation*          of a new class loader.** @since  1.2*/protected ClassLoader(ClassLoader parent) {this(checkCreateClassLoader(), parent);}/*** Creates a new class loader using the <tt>ClassLoader</tt> returned by* the method {@link #getSystemClassLoader()* <tt>getSystemClassLoader()</tt>} as the parent class loader.** <p> If there is a security manager, its {@link* SecurityManager#checkCreateClassLoader()* <tt>checkCreateClassLoader</tt>} method is invoked.  This may result in* a security exception.  </p>** @throws  SecurityException*          If a security manager exists and its*          <tt>checkCreateClassLoader</tt> method doesn't allow creation*          of a new class loader.*/protected ClassLoader() {this(checkCreateClassLoader(), getSystemClassLoader());}

常用方法

loadClass

    /*** Loads the class with the specified <a href="#name">binary name</a>.* This method searches for classes in the same manner as the {@link* #loadClass(String, boolean)} method.  It is invoked by the Java virtual* machine to resolve class references.  Invoking this method is equivalent* to invoking {@link #loadClass(String, boolean) <tt>loadClass(name,* false)</tt>}.** @param  name*         The <a href="#name">binary name</a> of the class** @return  The resulting <tt>Class</tt> object** @throws  ClassNotFoundException*          If the class was not found*          加载指定二进制名称的类*/public Class<?> loadClass(String name) throws ClassNotFoundException {return loadClass(name, false);}/*** Loads the class with the specified <a href="#name">binary name</a>.  The* default implementation of this method searches for classes in the* following order:** <ol>**   <li><p> Invoke {@link #findLoadedClass(String)} to check if the class*   has already been loaded.  </p></li>**   <li><p> Invoke the {@link #loadClass(String) <tt>loadClass</tt>} method*   on the parent class loader.  If the parent is <tt>null</tt> the class*   loader built-in to the virtual machine is used, instead.  </p></li>**   <li><p> Invoke the {@link #findClass(String)} method to find the*   class.  </p></li>** </ol>** <p> If the class was found using the above steps, and the* <tt>resolve</tt> flag is true, this method will then invoke the {@link* #resolveClass(Class)} method on the resulting <tt>Class</tt> object.** <p> Subclasses of <tt>ClassLoader</tt> are encouraged to override {@link* #findClass(String)}, rather than this method.  </p>** <p> Unless overridden, this method synchronizes on the result of* {@link #getClassLoadingLock <tt>getClassLoadingLock</tt>} method* during the entire class loading process.** @param  name*         The <a href="#name">binary name</a> of the class** @param  resolve*         If <tt>true</tt> then resolve the class*          如果为true则解析该类* @return  The resulting <tt>Class</tt> object** @throws  ClassNotFoundException*          If the class could not be found*/protected Class<?> loadClass(String name, boolean resolve)throws ClassNotFoundException{synchronized (getClassLoadingLock(name)) {// First, check if the class has already been loaded//校验class是否被加载Class<?> c = findLoadedClass(name);if (c == null) {long t0 = System.nanoTime();try {if (parent != null) {//递归，调用父类加载器的loadClassc = parent.loadClass(name, false);} else {//调用启动类加载器（虚拟机提供的加载器,即为BootStrapClassLoader）c = findBootstrapClassOrNull(name);}} catch (ClassNotFoundException e) {// ClassNotFoundException thrown if class not found// from the non-null parent class loader}if (c == null) {// If still not found, then invoke findClass in order// to find the class.long t1 = System.nanoTime();//父类加载器没有找到，再调用本身（这个本身包括ext和app）的findClass(name)来查找父类c = findClass(name);// this is the defining class loader; record the statssun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);sun.misc.PerfCounter.getFindClasses().increment();}}if (resolve) {//链接resolveClass(c);}return c;}}

getResource

    /*** Finds the resource with the given name.  A resource is some data* (images, audio, text, etc) that can be accessed by class code in a way* that is independent of the location of the code.** <p> The name of a resource is a '<tt>/</tt>'-separated path name that* identifies the resource.** <p> This method will first search the parent class loader for the* resource; if the parent is <tt>null</tt> the path of the class loader* built-in to the virtual machine is searched.  That failing, this method* will invoke {@link #findResource(String)} to find the resource.  </p>** @apiNote When overriding this method it is recommended that an* implementation ensures that any delegation is consistent with the {@link* #getResources(java.lang.String) getResources(String)} method.** @param  name*         The resource name** @return  A <tt>URL</tt> object for reading the resource, or*          <tt>null</tt> if the resource could not be found or the invoker*          doesn't have adequate  privileges to get the resource.** @since  1.1*  获得指定名称的资源   图像，音频，文本等*/public URL getResource(String name) {URL url;if (parent != null) {//搜索父类加载器的资源url = parent.getResource(name);} else {//不存在父类，搜索虚拟机默认的类加载器的路径，url = getBootstrapResource(name);}//仍然获取不到资源if (url == null) {//调用findResource(String)来查找资源url = findResource(name);}return url;}

双亲委派模型

提起Classs Loader 的类加载机制就不得不说说它的“双亲委派模型“

ClassLoader使用的是双亲委托模型来搜索类的，每个ClassLoader实例都有一个父类加载器的引用（不是继承的关系，是组合关系），虚拟机内置的启动类加载器（Bootstrap ClassLoader）本身没有父类加载器，但可以用作其它ClassLoader实例的的父类加载器。当一个ClassLoader实例需要加载某个类时，它会试图亲自搜索某个类之前，先把这个任务委托给它的父类加载器，这个过程是由上至下依次检查的，首先由最顶层的类加载器Bootstrap ClassLoader试图加载，如果没加载到，则把任务转交给Extension ClassLoader试图加载，如果也没加载到，则转交给App ClassLoader 进行加载，如果它也没有加载得到的话，则返回给委托的发起者，由它到指定的文件系统或网络等URL中加载该类。如果它们都没有加载到这个类时，则抛出ClassNotFoundException异常。否则将这个找到的类生成一个类的定义，并将它加载到内存当中，最后返回这个类在内存中的Class实例对象。

可以发现委托是从下向上，然后具体查找过程却是自上至下。

• 最高一层是BootStrap Class Loader

  它是在JVM 启动时候创建的，负载装载核心的Java 类, 比如Object , System String 等，主要位于jre/lib/rt.jar 中。

• 第二层是Platform ClassLoader

  即平台类加载器，负责装载扩展系统类，比如XMl, 加密，压缩相关的功能类，主要位于jre/lib/ext/*.jar

• 第三层类是Application ClassLoader

  即应用类加载器，主要加载用户自定义的ClassPath 路径下的类,主要位于ClassPath 中的jar.