说明
对于JDK源码分析的文章，仅仅记录我认为重要的地方。源码的细节实在太多，不可能面面俱到地写清每个逻辑。所以我的JDK源码分析，着重在JDK的体系架构层面，具体源码可以参考：http://www.cnblogs.com/skywang12345/category/455711.html。
例子
在开始研究异常的源码之前，首先看一下下面的示例：
@Test
public void testThrowable() {
try {
int a = 1 / 0;
} catch (Exception e) {
System.out.println(e.getMessage());
System.out.println(e.getCause());
e.printStackTrace();
}
}
输出
/ by zero
null
java.lang.ArithmeticException: / by zero
at test.jdk.testThrowable(jdk.java:383)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
...
调试信息
异常的框架
Throwable
Throwable 是所有 errors 和 exceptions 的父类，只有继承了 Throwable 才能被 Java 语句中的异常捕获。一个 Throwable 对象包含线程创建到执行至异常处的所有栈。同时也包括一个额外的 message 信息，来描述异常。
Exception
Exception 也并没有什么代码，基本上就是继承了 Throwable 而已！重要的是 Exception 可以被捕获，而 Error 不需要也不能被捕获！
Error
Error 表示非常严重的错误，一个正常的程序不应该 catch 这个错误。这些错误是「非检查异常」，在编译期是无法发现的。
Throwable 源码分析
最重要的成员：
private StackTraceElement[] stackTrace = UNASSIGNED_STACK;
其中一个StackTraceElement代表一个栈帧，最顶端的StackTraceElement代表异常抛出地点，其余每个StackTraceElement都表示一个方法调用。
源码如下：
/**
* An element in a stack trace, as returned by {@link
* Throwable#getStackTrace()}. Each element represents a single stack frame.
* All stack frames except for the one at the top of the stack represent
* a method invocation. The frame at the top of the stack represents the
* execution point at which the stack trace was generated. Typically,
* this is the point at which the throwable corresponding to the stack trace
* was created.
*
* @since 1.4
* @author Josh Bloch
*/
public final class StackTraceElement implements java.io.Serializable {
// Normally initialized by VM (public constructor added in 1.5)
private String declaringClass;
private String methodName;
private String fileName;
private int lineNumber;
UNASSIGNED_STACK 是一个空数组，用来表示未赋值的状态。
private static final StackTraceElement[] UNASSIGNED_STACK = new StackTraceElement[0];
主要看一个经常用到的用于打印堆栈信息的函数：printStackTrace。
private void printStackTrace(PrintStreamOrWriter s) {
// Guard against malicious overrides of Throwable.equals by
// using a Set with identity equality semantics.
Set<Throwable> dejaVu =
Collections.newSetFromMap(new IdentityHashMap<Throwable, Boolean>());
dejaVu.add(this);
synchronized (s.lock()) {
// Print our stack trace
s.println(this);
StackTraceElement[] trace = getOurStackTrace();
for (StackTraceElement traceElement : trace)
s.println("\tat " + traceElement);
// Print suppressed exceptions, if any
for (Throwable se : getSuppressed())
se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION, "\t", dejaVu);
// Print cause, if any
Throwable ourCause = getCause();
if (ourCause != null)
ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, "", dejaVu);
}
}
Exception 源码分析
Exception 没有什么代码，只是继承了Throwable而已！
/**
* The class {@code Exception} and its subclasses are a form of
* {@code Throwable} that indicates conditions that a reasonable
* application might want to catch.
*
* <p>The class {@code Exception} and any subclasses that are not also
* subclasses of {@link RuntimeException} are <em>checked
* exceptions</em>. Checked exceptions need to be declared in a
* method or constructor's {@code throws} clause if they can be thrown
* by the execution of the method or constructor and propagate outside
* the method or constructor boundary.
*
* @author Frank Yellin
* @see java.lang.Error
* @jls 11.2 Compile-Time Checking of Exceptions
* @since JDK1.0
*/
public class Exception extends Throwable {
Error 源码分析
Error 最重要的意义，在于 JVM 对它的约定。Error表示非常重要的错误，在编译期是无法检查出来的，程序不应该抛出Error。
/**
* An {@code Error} is a subclass of {@code Throwable}
* that indicates serious problems that a reasonable application
* should not try to catch. Most such errors are abnormal conditions.
* The {@code ThreadDeath} error, though a "normal" condition,
* is also a subclass of {@code Error} because most applications
* should not try to catch it.
* <p>
* A method is not required to declare in its {@code throws}
* clause any subclasses of {@code Error} that might be thrown
* during the execution of the method but not caught, since these
* errors are abnormal conditions that should never occur.
*
* That is, {@code Error} and its subclasses are regarded as unchecked
* exceptions for the purposes of compile-time checking of exceptions.
*
* @author Frank Yellin
* @see java.lang.ThreadDeath
* @jls 11.2 Compile-Time Checking of Exceptions
* @since JDK1.0
*/
public class Error extends Throwable {
总结
《Java编程思想》中提到：对于异常来说，最重要的是它的名字。异常的作用就是：用名字描述清楚所发生的错误，最好能够见名知意。用法也很简单：继承Exception即可！
