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层次结构和类图
- sun.misc.Launcher.ExtClassLoader
- sun.misc.Launcher.AppClassLoader
显式加载类
- this.getClass().getClassLoader().loadClass()
- Class.forName()
- MyClassLoader.findClass()
ClassLoader.loadClass()
ClassLoader.loadClass()的加载步骤为:
- 调用 来检查是否已经加载类。
- 在父类加载器上调用 方法。如果父类加载器为 null,则使用虚拟机的内置类加载器。
- 调用 方法查找类。
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 * <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 protected synchronized Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException // First, check if the class has already been loaded Class c = findLoadedClass(name); c = parent.loadClass(name, false); c = findBootstrapClassOrNull(name); } catch (ClassNotFoundException e) { // ClassNotFoundException thrown if class not found // from the non-null parent class loader // If still not found, then invoke findClass in order } URLClassLoader.findClass()
ClassLoader.loadClass()的最后一步是调用findClass(),这个方法在ClassLoader中并未实现,由其子类负责实现。
findClass()的功能是找到class文件并把字节码加载到内存中。
自定义的ClassLoader一般覆盖这个方法。——以便使用不同的加载路径。
/* The search path for classes and resources */ /* The context to be used when loading classes and resources */ private AccessControlContext acc; * Finds and loads the class with the specified name from the URL search * path. Any URLs referring to JAR files are loaded and opened as needed * until the class is found. * @param name the name of the class * @return the resulting class * @exception ClassNotFoundException if the class could not be found protected Class<?> findClass(final String name) throws ClassNotFoundException AccessController.doPrivileged(new PrivilegedExceptionAction() { public Object run() throws ClassNotFoundException { String path = name.replace('.', '/').concat(".class"); // 1. URLClassPath ucp,帮助获取class文件字节流 // URLClassPath会用FileLoader或者JarLoader去加载字节码 Resource res = ucp.getResource(path, false); // 2. defineClass,创建类对象,将字节流解析成JVM能够识别的Class对象。 return defineClass(name, res, true); } catch (IOException e) { throw new ClassNotFoundException(name, e); throw new ClassNotFoundException(name); } catch (java.security.PrivilegedActionException pae) { throw (ClassNotFoundException) pae.getException(); } ClassLoader.resolveClass()
* Links the specified class. This (misleadingly named) method may be * used by a class loader to link a class. If the class <tt>c</tt> has * already been linked, then this method simply returns. Otherwise, the * class is linked as described in the "Execution" chapter of the <a * href="http://java.sun.com/docs/books/jls/">Java Language Specification</a>. * @throws NullPointerException * If <tt>c</tt> is <tt>null</tt>. * @see #defineClass(String, byte[], int, int) protected final void resolveClass(Class<?> c) { private native void resolveClass0(Class c); 自定义加载器
findClass()的功能是找到class文件并把字节码加载到内存中。
自定义的ClassLoader一般覆盖这个方法。——以便使用不同的加载路径。
在其中调用defineClass()解析字节码。
自定义的加载器可以覆盖该方法loadClass(),以便定义不同的加载机制。
例如Servlet中的WebappClassLoader覆盖了该方法,在WEB-INFO/classes目录下查找类文件;在加载时,如果成功,则缓存到ResourceEntry对象。——不同的加载机制。
AppClassLoader覆盖了loadClass()方法。
如果自定义的加载器仅覆盖了findClass,而未覆盖loadClass(即加载规则一样,但加载路径不同);则调用getClass().getClassLoader()返回的仍然是AppClassLoader!因为真正load类的,还是AppClassLoader。
JVM默认不能热部署类,因为加载类时会去调用findLoadedClass(),如果类已被加载,就不会再次加载。
JVM判断类是否被加载有两个条件:完整类名是否一样、ClassLoader是否是同一个。
所以要实现热部署的话,只需要使用ClassLoader的不同实例来加载。
MyClassLoader cl1
= new MyClassLoader();
Class c1
= cl1.findClass(
"Test.class");
c1.newInstance();
MyClassLoader cl2
= new MyClassLoader();
Class c2
= cl2.findClass(
"Test.class");
上例中的c1和c2就是两个不同的实例。
如果用同一个ClassLoader实例来加载,则会抛LinkageError。
defineClass
* Converts an array of bytes into an instance of class <tt>Class</tt>. * Before the <tt>Class</tt> can be used it must be resolved. protected final Class<?> defineClass(String name, byte[] b, int off, int len) findLoadedClass
* Returns the class with the given <a href="#name">binary name</a> if this * loader has been recorded by the Java virtual machine as an initiating * loader of a class with that <a href="#name">binary name</a>. Otherwise * <tt>null</tt> is returned. </p> */ protected final Class<?> findLoadedClass(String name) findSystemClass
* Finds a class with the specified <a href="#name">binary name</a>, * loading it if necessary. * <p> This method loads the class through the system class loader (see * {@link #getSystemClassLoader()}). The <tt>Class</tt> object returned * might have more than one <tt>ClassLoader</tt> associated with it. * Subclasses of <tt>ClassLoader</tt> need not usually invoke this method, * because most class loaders need to override just {@link #findClass(String)}. </p> protected final Class<?> findSystemClass(String name) getParent
* Returns the parent class loader for delegation. Some implementations may * use <tt>null</tt> to represent the bootstrap class loader. This method * will return <tt>null</tt> in such implementations if this class loader's * parent is the bootstrap class loader. public final ClassLoader getParent() resolveClass
* Links the specified class. This (misleadingly named) method may be * used by a class loader to link a class. If the class <tt>c</tt> has * already been linked, then this method simply returns. Otherwise, the * class is linked as described in the "Execution" chapter of the <a * href="http://java.sun.com/docs/books/jls/">Java Language Specification</a>. */ protected final void resolveClass(Class<?> c)