来源在Java反射中，Class.forName和ClassLoader的区别
解释
在java中Class.forName()和ClassLoader都可以对类进行加载。ClassLoader就是遵循双亲委派模型最终调用启动类加载器的类加载器，实现的功能是“通过一个类的全限定名来获取描述此类的二进制字节流”，获取到二进制流后放到JVM中。Class.forName()方法实际上也是调用的CLassLoader来实现的。
Class.forName(String className)；这个方法的源码是：
@CallerSensitive
public static Class<?> forName(String className)
throws ClassNotFoundException {
Class<?> caller = Reflection.getCallerClass();
return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
}
最后调用的方法是forName0这个方法，在这个forName0方法中的第二个参数被默认设置为了true，这个参数代表是否对加载的类进行初始化，设置为true时会类进行初始化，代表会执行类中的静态代码块，以及对静态变量的赋值等操作。
也可以调用Class.forName(String name, boolean initialize,ClassLoader loader)方法来手动选择在加载类的时候是否要对类进行初始化。Class.forName(String name, boolean initialize,ClassLoader loader)的源码如下：
/**
* Returns the {@code Class} object associated with the class or
* interface with the given string name, using the given class loader.
* Given the fully qualified name for a class or interface (in the same
* format returned by {@code getName}) this method attempts to
* locate, load, and link the class or interface. The specified class
* loader is used to load the class or interface. If the parameter
* {@code loader} is null, the class is loaded through the bootstrap
* class loader. The class is initialized only if the
* {@code initialize} parameter is {@code true} and if it has
* not been initialized earlier.
*
* <p> If {@code name} denotes a primitive type or void, an attempt
* will be made to locate a user-defined class in the unnamed package whose
* name is {@code name}. Therefore, this method cannot be used to
* obtain any of the {@code Class} objects representing primitive
* types or void.
*
* <p> If {@code name} denotes an array class, the component type of
* the array class is loaded but not initialized.
*
* <p> For example, in an instance method the expression:
*
* <blockquote>
* {@code Class.forName("Foo")}
* </blockquote>
*
* is equivalent to:
*
* <blockquote>
* {@code Class.forName("Foo", true, this.getClass().getClassLoader())}
* </blockquote>
*
* Note that this method throws errors related to loading, linking or
* initializing as specified in Sections 12.2, 12.3 and 12.4 of <em>The
* Java Language Specification</em>.
* Note that this method does not check whether the requested class
* is accessible to its caller.
*
* <p> If the {@code loader} is {@code null}, and a security
* manager is present, and the caller's class loader is not null, then this
* method calls the security manager's {@code checkPermission} method
* with a {@code RuntimePermission("getClassLoader")} permission to
* ensure it's ok to access the bootstrap class loader.
*
* @param name fully qualified name of the desired class
* @param initialize if {@code true} the class will be initialized.
* See Section 12.4 of <em>The Java Language Specification</em>.
* @param loader class loader from which the class must be loaded
* @return class object representing the desired class
*
* @exception LinkageError if the linkage fails
* @exception ExceptionInInitializerError if the initialization provoked
* by this method fails
* @exception ClassNotFoundException if the class cannot be located by
* the specified class loader
*
* @see java.lang.Class#forName(String)
* @see java.lang.ClassLoader
* @since 1.2
*/
@CallerSensitive
public static Class<?> forName(String name, boolean initialize,
ClassLoader loader)
throws ClassNotFoundException
{
Class<?> caller = null;
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
// Reflective call to get caller class is only needed if a security manager
// is present. Avoid the overhead of making this call otherwise.
caller = Reflection.getCallerClass();
if (sun.misc.VM.isSystemDomainLoader(loader)) {
ClassLoader ccl = ClassLoader.getClassLoader(caller);
if (!sun.misc.VM.isSystemDomainLoader(ccl)) {
sm.checkPermission(
SecurityConstants.GET_CLASSLOADER_PERMISSION);
}
}
}
return forName0(name, initialize, loader, caller);
}
源码中的注释只摘取了一部分，其中对参数initialize的描述是：if {@code true} the class will be initialized.意思就是说：如果参数为true，则加载的类将会被初始化。
举例
下面还是举例来说明结果吧：
一个含有静态代码块、静态变量、赋值给静态变量的静态方法的类
public class ClassForName {
//静态代码块
static {
System.out.println("执行了静态代码块");
}
//静态变量
private static String staticFiled = staticMethod();
//赋值静态变量的静态方法
public static String staticMethod(){
System.out.println("执行了静态方法");
return "给静态字段赋值了";
}
}
测试方法：
public class MyTest {
@Test
public void test44(){
try {
Class.forName("com.test.mytest.ClassForName");
System.out.println("#########分割符(上面是Class.forName的加载过程，下面是ClassLoader的加载过程)##########");
ClassLoader.getSystemClassLoader().loadClass("com.test.mytest.ClassForName");
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
}
运行结果：
执行了静态代码块
执行了静态方法
#########分割符(上面是Class.forName的加载过程，下面是ClassLoader的加载过程)##########
根据运行结果得出Class.forName加载类是将类进了初始化，而ClassLoader的loadClass并没有对类进行初始化，只是把类加载到了虚拟机中。
应用场景
在我们熟悉的Spring框架中的IOC的实现就是使用的ClassLoader。
而在我们使用JDBC时通常是使用Class.forName()方法来加载数据库连接驱动。这是因为在JDBC规范中明确要求Driver(数据库驱动)类必须向DriverManager注册自己。
以MySQL的驱动为例解释：
public class Driver extends NonRegisteringDriver implements java.sql.Driver {
// ~ Static fields/initializers
// ---------------------------------------------
//
// Register ourselves with the DriverManager
//
static {
try {
java.sql.DriverManager.registerDriver(new Driver());
} catch (SQLException E) {
throw new RuntimeException("Can't register driver!");
}
}
// ~ Constructors
// -----------------------------------------------------------
/**
* Construct a new driver and register it with DriverManager
*
* @throws SQLException
* if a database error occurs.
*/
public Driver() throws SQLException {
// Required for Class.forName().newInstance()
}
}
我们看到Driver注册到DriverManager中的操作写在了静态代码块中，这就是为什么在写JDBC时使用Class.forName()的原因了。