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
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 SimpleException
2.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:)