本篇内容介绍了“Java的ClassLoader机制是什么”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
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JVM在加载类的时候，都是通过ClassLoader的loadClass()方法来加载class的，loadClass(String name)方法：
使用的是双亲委托模式：
jvm启动时，会启动jre/rt.jar里的类加载器：bootstrap classloader，用来加载java核心api；然后启动扩展类加载器ExtClassLoader加载扩展类，并加载用户程序加载器AppClassLoader，并指定ExtClassLoader为他的父类；
当类被加载时，会先检查在内存中是否已经被加载，如果是，则不再加载，如果没有，再由AppClassLoader来加载，先从jar包里找，没有再从classpath里找；
如果自定义loader类，就会存在这命名空间的情况，不同的加载器加载同一个类时，产生的实例其实是不同的；
Java代码
public Class loadClass(String name) throws ClassNotFoundException { return loadClass(name, false); } public Class loadClass(String name) throws ClassNotFoundException { return loadClass(name, false); }loadClass(String name)方法再调用loadClass(String name, boolean resolve)方法：
◆ name - 类的二进制名称
◆ resolve - 如果该参数为 true，则分析这个类
Java代码
protected synchronized Class loadClass(String name, boolean resolve) throws ClassNotFoundException { // First, check if the class has already been loaded //JVM 规范规定ClassLoader可以在缓存保留它所加载的Class，如果一个Class已经被加载过，则直接从缓存中获取 Class c = findLoadedClass(name); if (c == null) { try { if (parent != null) { c = parent.loadClass(name, false); } else { c = findBootstrapClass0(name); } } catch (ClassNotFoundException e) { // If still not found, then invoke findClass in order // to find the class. c = findClass(name); } } if (resolve) { resolveClass(c); } return c; } protected synchronized Class loadClass(String name, boolean resolve) throws ClassNotFoundException { // First, check if the class has already been loaded //JVM 规范规定ClassLoader可以在缓存保留它所加载的Class，如果一个Class已经被加载过，则直接从缓存中获取 Class c = findLoadedClass(name); if (c == null) { try { if (parent != null) { c = parent.loadClass(name, false); } else { c = findBootstrapClass0(name); } } catch (ClassNotFoundException e) { // If still not found, then invoke findClass in order // to find the class. c = findClass(name); } } if (resolve) { resolveClass(c); } return c; }如果ClassLoader并没有加载这个class，则调用findBootstrapClass0：
Java代码
private Class findBootstrapClass0(String name) throws ClassNotFoundException { check(); if (!checkName(name)) throw new ClassNotFoundException(name); return findBootstrapClass(name); } private Class findBootstrapClass0(String name) throws ClassNotFoundException { check(); if (!checkName(name)) throw new ClassNotFoundException(name); return findBootstrapClass(name); }该方法会调用check()方法来判断这个类是否已经初始化，并且通过checkName(name)来判断由name指定的这个类是否存在***调用findBootstrapClass(name)：
Java代码
private native Class findBootstrapClass(String name) throws ClassNotFoundException; private native Class findBootstrapClass(String name) throws ClassNotFoundException;
而这个findBootstrapClass方法是一个native方法，这是我们的root loader，这个载入方法并非是由JAVA所写，而是C++写的，它会最终调用JVM中的原生findBootstrapClass方法来完成类的加载。
如果上面两个都找不到，则使用findClass(name)来查找指定类名的Class：
Java代码
protected Class findClass(String name) throws ClassNotFoundException { throw new ClassNotFoundException(name); } protected Class findClass(String name) throws ClassNotFoundException { throw new ClassNotFoundException(name); }JDK5.0中的说明：
使用指定的二进制名称查找类。此方法应该被类加载器的实现重写，该实现按照委托模型来加载类。在通过父类加载器检查所请求的类后，此方法将被 loadClass 方法调用。默认实现抛出一个ClassNotFoundException。
所以，我们在自定义类中，只需要重写findClass()即可。
MyClassLoader类：
Java代码
public class MyClassLoader extends ClassLoader { private String fileName; public MyClassLoader(String fileName) { this.fileName = fileName; } protected Class findClass(String className) throws ClassNotFoundException { Class clazz = this.findLoadedClass(className); if (null == clazz) { try { String classFile = getClassFile(className); FileInputStream fis = new FileInputStream(classFile); FileChannel fileC = fis.getChannel(); ByteArrayOutputStream baos = new ByteArrayOutputStream(); WritableByteChannel outC = Channels.newChannel(baos); ByteBuffer buffer = ByteBuffer.allocateDirect(1024); while (true) { int i = fileC.read(buffer); if (i == 0 || i == -1) { break; } buffer.flip(); outC.write(buffer); buffer.clear(); } fis.close(); byte[] bytes = baos.toByteArray(); clazz = defineClass(className, bytes, 0, bytes.length); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } } return clazz; } private byte[] loadClassBytes(String className) throws ClassNotFoundException { try { String classFile = getClassFile(className); FileInputStream fis = new FileInputStream(classFile); FileChannel fileC = fis.getChannel(); ByteArrayOutputStream baos = new ByteArrayOutputStream(); WritableByteChannel outC = Channels.newChannel(baos); ByteBuffer buffer = ByteBuffer.allocateDirect(1024); while (true) { int i = fileC.read(buffer); if (i == 0 || i == -1) { break; } buffer.flip(); outC.write(buffer); buffer.clear(); } fis.close(); return baos.toByteArray(); } catch (IOException fnfe) { throw new ClassNotFoundException(className); } } private String getClassFile(String name) { StringBuffer sb = new StringBuffer(fileName); name = name.replace('.', File.separatorChar) + ".class"; sb.append(File.separator + name); return sb.toString(); } } public class MyClassLoader extends ClassLoader { private String fileName; public MyClassLoader(String fileName) { this.fileName = fileName; } protected Class findClass(String className) throws ClassNotFoundException { Class clazz = this.findLoadedClass(className); if (null == clazz) { try { String classFile = getClassFile(className); FileInputStream fis = new FileInputStream(classFile); FileChannel fileC = fis.getChannel(); ByteArrayOutputStream baos = new ByteArrayOutputStream(); WritableByteChannel outC = Channels.newChannel(baos); ByteBuffer buffer = ByteBuffer.allocateDirect(1024); while (true) { int i = fileC.read(buffer); if (i == 0 || i == -1) { break; } buffer.flip(); outC.write(buffer); buffer.clear(); } fis.close(); byte[] bytes = baos.toByteArray(); clazz = defineClass(className, bytes, 0, bytes.length); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } } return clazz; } private byte[] loadClassBytes(String className) throws ClassNotFoundException { try { String classFile = getClassFile(className); FileInputStream fis = new FileInputStream(classFile); FileChannel fileC = fis.getChannel(); ByteArrayOutputStream baos = new ByteArrayOutputStream(); WritableByteChannel outC = Channels.newChannel(baos); ByteBuffer buffer = ByteBuffer.allocateDirect(1024); while (true) { int i = fileC.read(buffer); if (i == 0 || i == -1) { break; } buffer.flip(); outC.write(buffer); buffer.clear(); } fis.close(); return baos.toByteArray(); } catch (IOException fnfe) { throw new ClassNotFoundException(className); } } private String getClassFile(String name) { StringBuffer sb = new StringBuffer(fileName); name = name.replace('.', File.separatorChar) + ".class"; sb.append(File.separator + name); return sb.toString(); } }该类中通过调用defineClass(String name, byte[] b, int off, int len)方法来定义一个类：
Java代码
protected final Class defineClass(String name, byte[] b, int off, int len) throws ClassFormatError { return defineClass(name, b, off, len, null); } protected final Class defineClass(String name, byte[] b, int off, int len) throws ClassFormatError { return defineClass(name, b, off, len, null); }注：MyClassLoader加载类时有一个局限，必需指定.class文件，而不能指定.jar文件。该类中的大部分代码是从网上搜索到的，是出自一牛人之笔，只是不知道原帖在哪，希望不会被隐藏。
MainClassLoader类：
Java代码
public class MainClassLoader { public static void main(String[] args) { try { MyClassLoader tc = new MyClassLoader("F:\\OpenLib\\"); Class c = tc.findClass("Test"); c.newInstance(); } catch (ClassNotFoundException e) { e.printStackTrace(); } catch (IllegalAccessException e) { e.printStackTrace(); } catch (InstantiationException e) { e.printStackTrace(); } } } public class MainClassLoader { public static void main(String[] args) { try { MyClassLoader tc = new MyClassLoader("F:\\OpenLib\\"); Class c = tc.findClass("Test"); c.newInstance(); } catch (ClassNotFoundException e) { e.printStackTrace(); } catch (IllegalAccessException e) { e.printStackTrace(); } catch (InstantiationException e) { e.printStackTrace(); } } }***是一个简单的Test测试类：
Java代码
public class Test { public Test() { System.out.println("Test"); } public static void main(String[] args) { System.out.println("Hello World"); } }“Java的ClassLoader机制是什么”的内容就介绍到这里了，感谢大家的阅读。如果想了解更多行业相关的知识可以关注创新互联网站，小编将为大家输出更多高质量的实用文章！
