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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#xff08;父优先的等级加载机制#xff09;
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…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 ConcurrentHashMapString, 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 GCed until the loader is GCed.// 记录当前类加载器加载的类private final VectorClass? 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 HashMapString, 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 weve loaded.private static VectorString loadedLibraryNames new Vector();// Native libraries belonging to system classes.private static VectorNativeLibrary systemNativeLibraries new Vector();// Native libraries associated with the class loader.private VectorNativeLibrary nativeLibraries new Vector();// native libraries being loaded/unloaded.private static StackNativeLibrary 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 ClassLoaders assertion status modification methods have// been invoked.// GuardedBy(assertionLock)private MapString, 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 ClassLoaders assertion status modification methods// have been invoked.// GuardedBy(assertionLock)MapString, 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()* ttcheckCreateClassLoader/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* ttcheckCreateClassLoader/tt method doesnt allow creation* of a new class loader.** since 1.2*/protected ClassLoader(ClassLoader parent) {this(checkCreateClassLoader(), parent);}/*** Creates a new class loader using the ttClassLoader/tt returned by* the method {link #getSystemClassLoader()* ttgetSystemClassLoader()/tt} as the parent class loader.** p If there is a security manager, its {link* SecurityManager#checkCreateClassLoader()* ttcheckCreateClassLoader/tt} method is invoked. This may result in* a security exception. /p** throws SecurityException* If a security manager exists and its* ttcheckCreateClassLoader/tt method doesnt 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#namebinary 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) ttloadClass(name,* false)/tt}.** param name* The a href#namebinary name/a of the class** return The resulting ttClass/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#namebinary name/a. The* default implementation of this method searches for classes in the* following order:** ol** lip Invoke {link #findLoadedClass(String)} to check if the class* has already been loaded. /p/li** lip Invoke the {link #loadClass(String) ttloadClass/tt} method* on the parent class loader. If the parent is ttnull/tt the class* loader built-in to the virtual machine is used, instead. /p/li** lip 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* ttresolve/tt flag is true, this method will then invoke the {link* #resolveClass(Class)} method on the resulting ttClass/tt object.** p Subclasses of ttClassLoader/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 ttgetClassLoadingLock/tt} method* during the entire class loading process.** param name* The a href#namebinary name/a of the class** param resolve* If tttrue/tt then resolve the class* 如果为true则解析该类* return The resulting ttClass/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BootStrapClassLoaderc 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 ttnull/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 ttURL/tt object for reading the resource, or* ttnull/tt if the resource could not be found or the invoker* doesnt 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.
