简单谈谈java的异常处理(Try Catch Finally)

异常的英文单词是exception,字面翻译就是“意外、例外”的意思,也就是非正常情况。事实上,异常本质上是程序上的错误,包括程序逻辑错误和系统错误。

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

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class SimpleException{

public void a() throws Exception{

throw new Exception();

};

}

2.捕获异常:

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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:

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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:

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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 SimpleException

2.抛出多个异常:

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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:

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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:

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/**

* 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:

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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:

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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:

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Process finished with exit code 0
别急,我们再来看点有趣的:

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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:

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---------------------------------------------------------------

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:

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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:

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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:

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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:

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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:

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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:

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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

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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:)

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