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：

使用的是雙親委托模式：

jvm啟動時，會啟動jre/rt.jar裡的類加載器：bootstrap classloader，用來加載Java核心api；然後啟動擴展類加載器ExtClassLoader加載擴展類，並加載用戶程序加載器AppClassLoader，並指定ExtClassLoader為他的父類；

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包裡找，沒有再從classpath裡找；

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 - 如果該參數為 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 規范規定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 規范規定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，該實現按照委托模型來加載類。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加載類時有一個局限，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"); 
} 
}