Java Exception
先贴上一段Exception源码注释
1 /** 2 * The class {@code Exception} and its subclasses are a form of 3 * {@code Throwable} that indicates conditions that a reasonable 4 * application might want to catch. 5 * 6 * <p>The class {@code Exception} and any subclasses that are not also 7 * subclasses of {@link RuntimeException} are <em>checked 8 * exceptions</em>. Checked exceptions need to be declared in a 9 * method or constructor's {@code throws} clause if they can be thrown 10 * by the execution of the method or constructor and propagate outside 11 * the method or constructor boundary. 12 * 13 * @author Frank Yellin 14 * @see java.lang.Error 15 * @jls 11.2 Compile-Time Checking of Exceptions 16 * @since JDK1.0 17 */
第一段能看出Exception与Error的不同，Exception是一个系统想要捕获的东西。（Errors是不应捕获的）
异常分为Checked Exception和Unchecked Exception。
CheckedException是除了RuntimeException外所有Exception及其子类，UncheckedException是RuntimeExcetion及其子类。
CheckedException需要使用throws处理，而UncheckedException无须使用throws处理。此处的Checked意思是Checked by coder。
UnCheckedException可以由compiler在运行propagate outside the method or constructor boundary,我们就不必处理了，如
IndexOutofBoundException,ArythmeticException。而FileNotFoundException,SqlBadSyntax之类的在编译阶段是没法处理的，因此
我们必须手动处理。
