Java的ClassLoader機制解析

JVM在加載類的時候，都是通過ClassLoader的loadClass()方法來加載class的，loadClass(String name)方法：

使用的是雙親委託模式：

jvm啓動時，會啓動jre/rt.jar裏的類加載器：bootstrap classloader，用來加載java核心api；然後啓動擴展類加載器ExtClassLoader加載擴展類，並加載用戶程序加載器AppClassLoader，並指定ExtClassLoader爲他的父類；

當類被加載時，會先檢查在內存中是否已經被加載，如果是，則不再加載，如果沒有，再由AppClassLoader來加載，先從jar包裏找，沒有再從classpath裏找；

如果自定義loader類，就會存在這命名空間的情況，不同的加載器加載同一個類時，產生的實例其實是不同的；

Java代碼

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)方法再調用loadClass(String name, boolean resolve)方法：

◆ name - 類的二進制名稱

◆ resolve - 如果該參數爲 true，則分析這個類

Java代碼

protected synchronized Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException { // First, check if the class has already been loaded //JVM 規範規定ClassLoader可以在緩存保留它所加載的Class，如果一個Class已經被加載過，則直接從緩存中獲取 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 規範規定ClassLoader可以在緩存保留它所加載的Class，如果一個Class已經被加載過，則直接從緩存中獲取 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並沒有加載這個class，則調用findBootstrapClass0：

Java代碼

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); } 該方法會調用check()方法來判斷這個類是否已經初始化，並且通過checkName(name)來判斷由name指定的這個類是否存在最後調用findBootstrapClass(name)：

Java代碼

private native Class findBootstrapClass(String name) throws ClassNotFoundException;  private native Class findBootstrapClass(String name) throws ClassNotFoundException; 而這個findBootstrapClass方法是一個native方法，這是我們的root loader，這個載入方法並非是由JAVA所寫，而是C++寫的，它會最終調用JVM中的原生findBootstrapClass方法來完成類的加載。

如果上面兩個都找不到，則使用findClass(name)來查找指定類名的Class：

Java代碼

protected Class<?> findClass(String name) throws ClassNotFoundException { throw new ClassNotFoundException(name); }  protected Class<?> findClass(String name) throws ClassNotFoundException { throw new ClassNotFoundException(name); } JDK5.0中的說明：

使用指定的二進制名稱查找類。此方法應該被類加載器的實現重寫，該實現按照委託模型來加載類。在通過父類加載器檢查所請求的類後，此方法將被 loadClass 方法調用。默認實現拋出一個ClassNotFoundException。

所以，我們在自定義類中，只需要重寫findClass()即可。

MyClassLoader類：

Java代碼

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(); } } 該類中通過調用defineClass(String name, byte[] b, int off, int len)方法來定義一個類：

Java代碼

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加載類時有一個侷限，必需指定.class文件，而不能指定.jar文件。該類中的大部分代碼是從網上搜索到的，是出自一牛人之筆，只是不知道原帖在哪，希望不會被隱藏。

MainClassLoader類：

Java代碼

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(); } } } 最後是一個簡單的Test測試類：

Java代碼

public class Test { public Test() { System.out.println("Test"); } public static void main(String[] args) { System.out.println("Hello World"); } }