Class.forName(String className);这个方法的源码是:
@CallerSensitive
public static Class<?> forName(String className)
throws ClassNotFoundException {
return forName(className, true, VMStack.getCallingClassLoader());
}
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 whether the class must be initialized
* @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
{
if (loader == null) {
loader = BootClassLoader.getInstance();
}
Class<?> result;
try {
result = classForName(name, initialize, loader);
} catch (ClassNotFoundException e) {
Throwable cause = e.getCause();
if (cause instanceof LinkageError) {
throw (LinkageError) cause;
}
throw e;
}
return result;
}
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 test() {
try {
class.forName("com.test.mytest.ClassForName");
System.out.println("#####分隔符(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
#####分隔符(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/initiallzers
// --------------------------------
// Register ourselves with the DriverManager
static {
try {
java.sql.DriverManager.registerDriver(new Driver());
} catch (SQLException e) {
throw new RuntimeException("Can't regoster driver!");
}
}
// Constuctors
/**
* 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。
来源:http://t.cn/AiQQ7dwi