一 前言
java异常处理大家都不陌生，总的来说有下面两点：
1.抛出异常:throw exception
class SimpleException{
public void a() throws Exception{
throw new Exception();
};
}2.捕获异常：
public class MyException {
public static void main(String[] args){
MyException e = new MyException();
SimpleException se = new SimpleException();
try {
se.a();
} catch (Exception e1) {
e1.printStackTrace();
}
}
}
class SimpleException{
public void a() throws Exception{
throw new Exception();
};
｝
本文将在此基础上，更加深入的谈一些细节问题。
二 自定义异常类
java语言为我们提供了很多异常类，但是有时候我们为了写代码的方便还是要自定义的去创造异常类：
class SimpleException extends Exception {};创建好之后我们可以使用try catch捕获它：
public class MyException {
public static void main(String[] args){
MyException e = new MyException();
try {
e.a();
} catch (SimpleException e1) {
e1.printStackTrace();
}
}
public void a() throws SimpleException{
throw new SimpleException();
}
}
class SimpleException extends Exception {};我们在MyException中定义了一个方法a()，让它抛出SimpleException异常，然后我们在main()中调用这个方法，并使用try catch捕获了这个异常：
SimpleException
at MyException.a(MyException.java:15)
at MyException.main(MyException.java:8)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
Process finished with exit code 0编译执行后的结果，主要看前三行就行了。这里着重说明几点：
1.抛出异常类型的指定：(exception specification)
当我们需要在一个方法中抛出一个异常时，我们使用throw后加某异常类的实例，程序会在此向客户端程序（调用这段代码的程序）抛出对应异常并在此退出（相当于return）。另外需要注意的是，我们必须在定义该方法的时候指明异常类型，比如下面这段代码会抛出SimpleException异常
public void a() throws SimpleException2.抛出多个异常：
public void a() throws SimpleException,AException,BException{
throw new SimpleException();
}不同的异常类之间用逗号隔开即可，在这种情况下我们不必须throw每个异常类的实例（），但是客户端代码必须要catch到每个异常类：
public class MyException {
public static void main(String[] args){
MyException e = new MyException();
try {
e.a();
} catch (SimpleException e1) {
e1.printStackTrace();
} catch (BException e1) {
e1.printStackTrace();
} catch (AException e1) {
e1.printStackTrace();
}
}
public void a() throws SimpleException,AException,BException{
throw new SimpleException();
}
}
class SimpleException extends Exception {};
class AException extends Exception{}
class BException extends Exception{}
三 stack trace
无论是抛出异常，或者是捕获处理异常，我们的目的是为了写出更健壮的程序，这很大程度上依赖于java异常机制给我们提供的异常信息，而它的载体就是stack trace。
前面的代码中我们直接使用printStackTrace()打印出异常信息，其实我们还可以使用getStackTrace()方法来获取StackTraceElement型的集合，如果你手头有IDEA的话，你可以先搜索出StackTraceElement类，可以发现它实现了接口Serializable ，再看看它的类描述：
/**
* 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
*/讲的很清楚，这个类的每个实例都是stack trace的一个元素，代表着一个stack frame，stack trace是由getStackTrace()方法返回的。后边的我试着翻译了几遍，都觉得不好，还是直接上代码才能说清楚：
public class MyException {
public static void main(String[] args){
MyException e = new MyException();
e.a();
public void a(){
try {
throw new Exception();
} catch (Exception e) {
StackTraceElement[] ste = e.getStackTrace();
System.out.println(ste.length);
}
}
}我们定义了方法a，让它抛出Exception异常的同时捕获它，然后我们通过getStackTrace()方法得到一个StackTraceElement型的数组，并打印出数组的长度：
7
Process finished with exit code 0我们把代码稍微改一下，不在a中捕获异常了，我们重新定义一个方法b，让它在调用a的同时将异常捕获：
public class MyException {
public static void main(String[] args){
MyException e = new MyException();
e.b();
}
public void b(){
try {
a();
} catch (Exception e) {
StackTraceElement[] ste = e.getStackTrace();
System.out.println(ste.length);
}
}
public void a() throws Exception{
throw new Exception();
}
}结果如下：
8
Process finished with exit code 0别急，我们再来看点有趣的：
public class MyException {
public static void main(String[] args){
MyException exception = new MyException();
try {
exception.c();
} catch (Exception e) {
StackTraceElement[] ste = e.getStackTrace();
System.out.println(ste.length);
System.out.println("---------------------------------------------------------------");
for (StackTraceElement s : e.getStackTrace()){
System.out.println(s.getClassName()+":method "+s.getMethodName()+" at line"+s.getLineNumber());
}
System.out.println("---------------------------------------------------------------");
}
}
public void c() throws Exception{
try {
a();
}catch (Exception e){
throw e;
}
}
public void a() throws Exception{
throw new Exception();
}
}下面是结果：
8
---------------------------------------------------------------
MyException:method a at line43
MyException:method c at line39
MyException:method main at line9
sun.reflect.NativeMethodAccessorImpl:method invoke0 at line-2
sun.reflect.NativeMethodAccessorImpl:method invoke at line57
sun.reflect.DelegatingMethodAccessorImpl:method invoke at line43
java.lang.reflect.Method:method invoke at line606
com.intellij.rt.execution.application.AppMain:method main at line144
---------------------------------------------------------------
Process finished with exit code 0也就是说，getStackTrace()返回一个栈，它包含从调用者(main())到初始抛出异常者(a())的一些基本信息 ,在上面的代码中，我们在c方法中调用a方法时捕获异常并通过throws将其再次抛出(rethrow)，调用c方法的方法可以捕获并处理异常，也可以选择继续抛出让更高层次的调用者（靠近栈底）处理。rethrow虽然很方便，但存在着一些问题，我们看下面这段代码：
public class MyException {
public static void main(String[] args){
MyException exception = new MyException();
try {
exception.c();
} catch (Exception e) {
e.printStackTrace(System.out);
}
}
public void c() throws Exception{
try {
a();
}catch (Exception e){
throw e;
}
}
public void a() throws Exception{
throw new Exception("Exception from a()");
}
}
java.lang.Exception: Exception from a()
at MyException.a(MyException.java:40)
at MyException.c(MyException.java:30)
at MyException.main(MyException.java:21)我们在c中重新抛出e，在main中使用 e.printStackTrace()打印出来，可以看到打印出来stack trace还是属于a的，如果我们想把stack trace变成c的可以这么写：
public class MyException {
public static void main(String[] args){
MyException exception = new MyException();
try {
exception.c();
} catch (Exception e) {
e.printStackTrace(System.out);
}
}
public void c() throws Exception{
try {
a();
}catch (Exception e){
// throw e;
throw (Exception)e.fillInStackTrace();
}
}
public void a() throws Exception{
throw new Exception("Exception from a()");
}
}java.lang.Exception: Exception from a()
at MyException.c(MyException.java:22)
at MyException.main(MyException.java:10)四 异常链 Exception chaining
先来看一个场景：
public class TestException {
public static void main(String[] args){
TestException testException = new TestException();
try {
testException.c();
} catch (CException e) {
e.printStackTrace();
}
}
public void a() throws AException{
AException aException = new AException("this is a exception");
throw aException;
}
public void b() throws BException{
try {
a();
} catch (AException e) {
throw new BException("this is b exception");
}
}
public void c() throws CException{
try {
b();
} catch (BException e) {
throw new CException("this is c exception");
}
}
}
class AException extends Exception{
public AException(String msg){
super(msg);
}
}
class BException extends Exception{
public BException(String msg){
super(msg);
}
}
class CException extends Exception{
public CException(String msg){
super(msg);
}
}创建了三个异常类AException、BException、CException，然后在a()中抛出AException，在b()中捕获AException并抛出BException，最后在c()中捕获BException并抛出CException，结果打印如下：
CException: this is c exception
at TestException.c(TestException.java:31)
at TestException.main(TestException.java:8)好，我们只看到了CException的信息，AException，BException的异常信息已丢失，这时候异常链的作用就出来了，看代码：
public class TestException {
public static void main(String[] args){
TestException testException = new TestException();
try {
testException.c();
} catch (CException e) {
e.printStackTrace();
}
}
public void a() throws AException{
AException aException = new AException("this is a exception");
throw aException;
}
public void b() throws BException{
try {
a();
} catch (AException e) {
// throw new BException("this is b exception");
BException bException = new BException("this is b exception");
bException.initCause(e);
throw bException;
}
}
public void c() throws CException{
try {
b();
} catch (BException e) {
// throw new CException("this is c exception");
CException cException = new CException("this is c exception");
cException.initCause(e);
throw cException;
}
}
}
class AException extends Exception{
public AException(String msg){
super(msg);
}
}
class BException extends Exception{
public BException(String msg){
super(msg);
}
}
class CException extends Exception{
public CException(String msg){
super(msg);
}
}我们用initCause()方法将异常信息给串联了起来，结果如下：
CException: this is c exception
at TestException.c(TestException.java:35)
at TestException.main(TestException.java:8)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
Caused by: BException: this is b exception
at TestException.b(TestException.java:24)
at TestException.c(TestException.java:32)
... 6 more
Caused by: AException: this is a exception
at TestException.a(TestException.java:15)
at TestException.b(TestException.java:21)
... 7 more
Process finished with exit code 0五 后记
其实关于java异常处理还有很多需要探讨的地方，但是由于我经验有限，还不能体会的太深刻，最常用的也就是
try {
...
}catch (Exception e){
...
}finally {
//不管异常会不会被捕捉或者处理都会执行的代码，如关闭IO操作
}但是无论如何我们还是要感谢java给我们提供的异常机制，它好似一个长者，时不时给我们指引道路，也让我们在编码的时候没有那么无聊：）