1.类加载器作用
①将.class文件加载到jvm ②将class文件统一编码成jvm要求格式
2. 类加载过程
初始化时机:
a)创建类的实例
b)对类进行反射调用,如果该类没有没初始化则需要先触发其初始化
c)初始化一个类的时候,如果其父类没有初始化,需要先对其父类进行初始化
d)当虚拟机启动时,用户需要一个执行主类,这个主类会初始化
类实例化VS类初始化
类实例化是创建一个类的实例对象
类初始化是对类的成员变量(static修饰的变量)赋初始值的过程
3.类加载机制
启动类(bootstrap classLoader):jre/bin目录下jar 对应开发项目中JRE System
扩展类(extension classLoader):jre/bin/ext目录下jar 对应开发项目中JRE System
应用类(application classLoader):编码者自己编码目录,对应开发项目中
4、ClassLoader类加载原理(双亲委派)
除了启动类加载器(bootstrap classloader)都有父类加载器。当一个classloader实例需要加载某个类时,会把任务委托给父类加载器,如果父类加载器加载失败会委托给启动类加载器,如果都失败才会在该类加载器中进行类加载。最后如果都没有加载到这个类,则抛出classnotfoundexception
5、双亲委派好处
①避免重复加载,父类加载后,classloader不需要再加载一次
②安全因素,双亲委托的方式,就可以避免自己随机定义的String来代替java核心api,因为String已经在启动时就被引导类加载器(Bootstrcp ClassLoader)加载,所以用户自定义的ClassLoader永远也无法加载一个自己写的String
6、classLoader代码
protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException { synchronized (getClassLoadingLock(name)) { // First, check if the class has already been loaded Class<?> c = findLoadedClass(name); if (c == null) { long t0 = System.nanoTime(); try { if (parent != null) { c = parent.loadClass(name, false); //递归方法,一直寻找到根节点处父加载器 } else { 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(); c = findClass(name); // this is the defining class loader; record the stats sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0); sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1); sun.misc.PerfCounter.getFindClasses().increment(); } } if (resolve) { resolveClass(c); } return c; } } protected final Class<?> findLoadedClass(String name) { if (!checkName(name)) return null; return findLoadedClass0(name); }
代码流程:
代码注意点:①synchronized (getClassLoadingLock(name))
分析getClassLoadingLock():
a: getclassLoadingLock()方法:
protected Object getClassLoadingLock(String className) { Object lock = this; if (parallelLockMap != null) { Object newLock = new Object(); lock = parallelLockMap.putIfAbsent(className, newLock); if (lock == null) { lock = newLock; } } return lock; }
b: parallelLockMap定义:
private final ConcurrentHashMap<String, Object> parallelLockMap;
parallelLockMap初始化在classLoader中:
private ClassLoader(Void unused, ClassLoader parent) { this.parent = parent; if (ParallelLoaders.isRegistered(this.getClass())) { parallelLockMap = new ConcurrentHashMap<>(); package2certs = new ConcurrentHashMap<>(); domains = Collections.synchronizedSet(new HashSet<ProtectionDomain>()); assertionLock = new Object(); } else { // no finer-grained lock; lock on the classloader instance parallelLockMap = null; package2certs = new Hashtable<>(); domains = new HashSet<>(); assertionLock = this; } }
c: 根据ParallelLoaders
的Registered
状态的不同来给parallelLockMap
赋值
② findLoadedClass(name) 首先从jvm内存中查找该类,如果不存在再去加载
③ parent.loadClass(name, false);多次递归调用该方法,用以发现根节点的父类加载器