反射获取对象示例的方式及个方式对比（附源码解析）
四种实现方式
-
对象.getClass()
// 对象都有了还要反射干什么。所以一般不用这个
// 不会初始化对象 Class.forName()
// 使用最多的方式
// 会初始化对象-
对象类.class 属性
// 需要导入类的包，依赖太强
// 对象都已经存在 肯定 已经初始化 -
通过ClassLoader
// 不会对 对象初始化
实现源码及各方法对比
@Test
public void getClassTest() throws ClassNotFoundException {
// 获取Class 对象 的方式
// 1. 对象.getClass();
// 对象都有了还要反射干什么
// 不会初始化对象
People p = new People();
Class<? extends People> aClass = p.getClass();
System.out.println(aClass);
// 2. Class.forName()
// 使用最多的方式
// 会初始化对象
Class<?> aClass1 = Class.forName("com.studynote.utils.People");
System.out.println(aClass1);
// 可以控制是否 初始化对象 并且可以指定 类加载器
Class<?> class1 = Class.forName("com.studynote.utils.People", true, this.getClass().getClassLoader());
// 3. 对象类.class 属性
// 需要导入类的包，依赖太强
// 对象都已经存在 肯定 已经初始化
Class<People> pClass = People.class;
System.out.println(pClass);
// 4. 通过ClassLoader
// 不会对 对象初始化
Class<?> aClass2 = ClassLoader.getSystemClassLoader().loadClass("com.studynote.utils.People");
System.out.println(aClass2);
System.out.println("+++++++++++++++++++++++++++++++++++++++++");
// getClasses() 返回该类中所有 公共 类和接口类的对象数组
Arrays.stream(aClass1.getClasses()).forEach(System.out::println);
// getDeclaredClasses() 返回该类中所有类和接口类的对象数组
Arrays.stream(aClass1.getDeclaredClasses()).forEach(System.out::println);
}
1. clazz.forName(String name, boolean initialize, ClassLoader loader) 源码
/**
* 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.
*
* @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
* @exception SecurityException
* if a security manager is present, and the {@code loader} is
* {@code null}, and the caller's class loader is not
* {@code null}, and the caller does not have the
* {@link RuntimePermission}{@code ("getClassLoader")}
*
* @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 (loader == null) {
ClassLoader ccl = ClassLoader.getClassLoader(caller);
if (ccl != null) {
sm.checkPermission(
SecurityConstants.GET_CLASSLOADER_PERMISSION);
}
}
}
return forName0(name, initialize, loader, caller);
}
2. clazz.forName(String className)源码解析
@CallerSensitive
public static Class<?> forName(String className)
throws ClassNotFoundException {
Class<?> caller = Reflection.getCallerClass();
return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
}
3.ClassLoader.loadClass(String name) 源码解析
/**
* Loads the class with the specified <a href="#binary-name">binary name</a>.
* This method searches for classes in the same manner as the {@link
* #loadClass(String, boolean)} method. It is invoked by the Java virtual
* machine to resolve class references. Invoking this method is equivalent
* to invoking {@link #loadClass(String, boolean) loadClass(name,
* false)}.
*
* @param name
* The <a href="#binary-name">binary name</a> of the class
*
* @return The resulting {@code Class} object
*
* @throws ClassNotFoundException
* If the class was not found
*/
public Class<?> loadClass(String name) throws ClassNotFoundException {
return loadClass(name, false);
}
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