Java的基本概念是结构不佳的代码不能运行。
12.1概念
异常处理把“描述在正常执行过程中做什么事”的代码和“出现问题怎么办”的代码相分离。
12.2基本异常
异常是在当前环境下无法获得必要的信息来解决问题。
异常抛出后：
将用new在异常堆上创建对象。
当前执行路径被终止，从当前环境中弹出对异常对象的引用。
异常处理程序接管程序，从恰当的地方（异常处理程序）继续执行程序。程序从错误中恢复过来，继续执行。
抛出一个异常：
if(t == null) throw NullPointerException();
异常可以是我们将每件事都当一个事物来考虑。
使用new创建异常对象后，此对象的引用将传递给throw。
能够抛出任意类型的Throwable对象，是异常类型的跟类。
12.3捕获异常
监控区域：一段能产生异常的代码，后面跟着处理这些异常的代码。
如果在方法内部抛出了异常，这个方法将在抛出异常的过程中结束。用Try块来捕获异常：
try{}
异常处理程序在catch之后。
try{
}catch(Type1 id1){
}catch(Type2 id2){
}异常处理机制将负责搜寻参数与异常类型匹配的第一个处理程序，进入catch执行后，异常得到处理。
java支持终止模型，一旦异常被抛出，就表明错误已无法挽回，也不能回来继续执行。
恢复模型难以实现。
12.4创建自定义异常
要自己定义异常类，必须从已有的异常类继承：
//: exceptions/InheritingExceptions.java
// Creating your own exceptions.
class SimpleException extends Exception {}
public class InheritingExceptions {
public void f() throws SimpleException {
System.out.println("Throw SimpleException from f()");
throw new SimpleException();
}
public static void main(String[] args) {
InheritingExceptions sed = new InheritingExceptions();
try {
sed.f();
} catch(SimpleException e) {
System.out.println("Caught it!");
}
}
} /* Output:
Throw SimpleException from f()
Caught it!
*///:~
也可以为异常类定义一个接受字符串参数的构造器：
//: exceptions/FullConstructors.java
//自己注释，自己创建的异常类，一个有参构造器，一个无参构造器
class MyException extends Exception {
public MyException() {}
public MyException(String msg) { super(msg); }
}
public class FullConstructors {
public static void f() throws MyException {
System.out.println("Throwing MyException from f()");
throw new MyException();
}
public static void g() throws MyException {
System.out.println("Throwing MyException from g()");
throw new MyException("Originated in g()");
}
public static void main(String[] args) {
try {
f();
} catch(MyException e) {
e.printStackTrace(System.out);
}
try {
g();
} catch(MyException e) {
e.printStackTrace(System.out);
}
}
} /* Output:
Throwing MyException from f()
MyException
at FullConstructors.f(FullConstructors.java:11)
at FullConstructors.main(FullConstructors.java:19)
Throwing MyException from g()
MyException: Originated in g()
at FullConstructors.g(FullConstructors.java:15)
at FullConstructors.main(FullConstructors.java:24)
*///:~
如果调用e.printStackTrace(),信息被输出到标准错误流中。本例中被发送到System.out中。
基本的日志记录功能：
//: exceptions/LoggingExceptions.java
// An exception that reports through a Logger.
import java.util.logging.*;
import java.io.*;
class LoggingException extends Exception {
private static Logger logger =
Logger.getLogger("LoggingException");
public LoggingException() {
StringWriter trace = new StringWriter();
printStackTrace(new PrintWriter(trace));
logger.severe(trace.toString());
}
}
public class LoggingExceptions {
public static void main(String[] args) {
try {
throw new LoggingException();
} catch(LoggingException e) {
System.err.println("Caught " + e);
}
try {
throw new LoggingException();
} catch(LoggingException e) {
System.err.println("Caught " + e);
}
}
} /* Output: (85% match)
Aug 30, 2005 4:02:31 PM LoggingException <init>
SEVERE: LoggingException
at LoggingExceptions.main(LoggingExceptions.java:19)
Caught LoggingException
Aug 30, 2005 4:02:31 PM LoggingException <init>
SEVERE: LoggingException
at LoggingExceptions.main(LoggingExceptions.java:24)
Caught LoggingException
*///:~
静态的Logger.getLogger创建了一个String参数相关联的Logger对象。
更常见的情形是我们需要捕获和记录别人编写的异常，必须在异常处理程序中生成日志消息：
//: exceptions/LoggingExceptions2.java
// Logging caught exceptions.
import java.util.logging.*;
import java.io.*;
public class LoggingExceptions2 {
private static Logger logger =
Logger.getLogger("LoggingExceptions2");
static void logException(Exception e) {
StringWriter trace = new StringWriter();
e.printStackTrace(new PrintWriter(trace));
logger.severe(trace.toString());
}
public static void main(String[] args) {
try {
throw new NullPointerException();
} catch(NullPointerException e) {
logException(e);
}
}
} /* Output: (90% match)
Aug 30, 2005 4:07:54 PM LoggingExceptions2 logException
SEVERE: java.lang.NullPointerException
at LoggingExceptions2.main(LoggingExceptions2.java:16)
*///:~
还可以更近一步自定义异常，比如加入额外的构造器和成员：
//: exceptions/ExtraFeatures.java
// Further embellishment of exception classes.
import static net.mindview.util.Print.*;
class MyException2 extends Exception {
private int x;
public MyException2() {}
public MyException2(String msg) { super(msg); }
public MyException2(String msg, int x) {
super(msg);
this.x = x;
}
public int val() { return x; }
public String getMessage() {
return "Detail Message: "+ x + " "+ super.getMessage();
}
}
public class ExtraFeatures {
public static void f() throws MyException2 {
print("Throwing MyException2 from f()");
throw new MyException2();
}
public static void g() throws MyException2 {
print("Throwing MyException2 from g()");
throw new MyException2("Originated in g()");
}
public static void h() throws MyException2 {
print("Throwing MyException2 from h()");
throw new MyException2("Originated in h()", 47);
}
public static void main(String[] args) {
try {
f();
} catch(MyException2 e) {
e.printStackTrace(System.out);
}
try {
g();
} catch(MyException2 e) {
e.printStackTrace(System.out);
}
try {
h();
} catch(MyException2 e) {
e.printStackTrace(System.out);
System.out.println("e.val() = " + e.val());
}
}
} /* Output:
Throwing MyException2 from f()
MyException2: Detail Message: 0 null
at ExtraFeatures.f(ExtraFeatures.java:22)
at ExtraFeatures.main(ExtraFeatures.java:34)
Throwing MyException2 from g()
MyException2: Detail Message: 0 Originated in g()
at ExtraFeatures.g(ExtraFeatures.java:26)
at ExtraFeatures.main(ExtraFeatures.java:39)
Throwing MyException2 from h()
MyException2: Detail Message: 47 Originated in h()
at ExtraFeatures.h(ExtraFeatures.java:30)
at ExtraFeatures.main(ExtraFeatures.java:44)
e.val() = 47
*///:~
新的异常添加了字段x已经设定x值的构造器和读取数据的方法。
既然异常也是对象，可以继续修改这个异常类，得到更强大的功能。但一般用不上。
12.5异常说明
异常说明：属于方法声明的一部分，紧跟在形式参数列表之后。
void f() Toobig,TooSmall{}
要么处理异常，要么就得在方法说明中表明方法将产生异常。
12.6捕获所有异常
以可只写一个异常处理程序来捕获所有异常。可以通过捕获异常类的基类Exception实现。
catch(Exception e){}
将捕获所有异常，所以应该放在处理程序最后。
//: exceptions/ExceptionMethods.java
// Demonstrating the Exception Methods.
import static net.mindview.util.Print.*;
public class ExceptionMethods {
public static void main(String[] args) {
try {
throw new Exception("My Exception");
} catch(Exception e) {
print("Caught Exception");
print("getMessage():" + e.getMessage());
print("getLocalizedMessage():" +
e.getLocalizedMessage());
print("toString():" + e);
print("printStackTrace():");
e.printStackTrace(System.out);
}
}
} /* Output:
Caught Exception
getMessage():My Exception
getLocalizedMessage():My Exception
toString():java.lang.Exception: My Exception
printStackTrace():
java.lang.Exception: My Exception
at ExceptionMethods.main(ExceptionMethods.java:8)
*///:~
栈轨迹：
printStackTrace()方法提供的信息可以通过getStackTrace()，这个方法将返回一个由栈轨迹中的元素所构成的数组，其中每一个元素都表示栈中的一帧。
//: exceptions/WhoCalled.java
// Programmatic access to stack trace information.
public class WhoCalled {
static void f() {
// Generate an exception to fill in the stack trace
try {
throw new Exception();
} catch (Exception e) {
for(StackTraceElement ste : e.getStackTrace())
System.out.println(ste.getMethodName());
}
}
static void g() { f(); }
static void h() { g(); }
public static void main(String[] args) {
f();
System.out.println("--------------------------------");
g();
System.out.println("--------------------------------");
h();
}
} /* Output:
f
main
--------------------------------
f
g
main
--------------------------------
f
g
h
main
*///:~
有时候希望把刚捕获的异常重新抛出，尤其是在用Exception捕获所有异常的时候。既然已经得到了对当前异常对象的引用，就可以直接把他们抛出：
catch（Exception e）{
System.out.println("抛出异常");
throw e;
}
//: exceptions/Rethrowing.java
// Demonstrating fillInStackTrace()
public class Rethrowing {
public static void f() throws Exception {
System.out.println("originating the exception in f()");
throw new Exception("thrown from f()");
}
public static void g() throws Exception {
try {
f();
} catch(Exception e) {
System.out.println("Inside g(),e.printStackTrace()");
e.printStackTrace(System.out);
throw e;
}
}
public static void h() throws Exception {
try {
f();
} catch(Exception e) {
System.out.println("Inside h(),e.printStackTrace()");
e.printStackTrace(System.out);
throw (Exception)e.fillInStackTrace();
}
}
public static void main(String[] args) {
try {
g();
} catch(Exception e) {
System.out.println("main: printStackTrace()");
e.printStackTrace(System.out);
}
try {
h();
} catch(Exception e) {
System.out.println("main: printStackTrace()");
e.printStackTrace(System.out);
}
}
} /* Output:
originating the exception in f()
Inside g(),e.printStackTrace()
java.lang.Exception: thrown from f()
at Rethrowing.f(Rethrowing.java:7)
at Rethrowing.g(Rethrowing.java:11)
at Rethrowing.main(Rethrowing.java:29)
main: printStackTrace()
java.lang.Exception: thrown from f()
at Rethrowing.f(Rethrowing.java:7)
at Rethrowing.g(Rethrowing.java:11)
at Rethrowing.main(Rethrowing.java:29)
originating the exception in f()
Inside h(),e.printStackTrace()
java.lang.Exception: thrown from f()
at Rethrowing.f(Rethrowing.java:7)
at Rethrowing.h(Rethrowing.java:20)
at Rethrowing.main(Rethrowing.java:35)
main: printStackTrace()
java.lang.Exception: thrown from f()
at Rethrowing.h(Rethrowing.java:24)
at Rethrowing.main(Rethrowing.java:35)
*///:~
调用fullInStrckTrace()的哪一行就成了异常的新发生地了。
可能在捕获异常后抛出一种新的异常：
//: exceptions/RethrowNew.java
// Rethrow a different object from the one that was caught.
class OneException extends Exception {
public OneException(String s) { super(s); }
}
class TwoException extends Exception {
public TwoException(String s) { super(s); }
}
public class RethrowNew {
public static void f() throws OneException {
System.out.println("originating the exception in f()");
throw new OneException("thrown from f()");
}
public static void main(String[] args) {
try {
try {
f();
} catch(OneException e) {
System.out.println(
"Caught in inner try, e.printStackTrace()");
e.printStackTrace(System.out);
throw new TwoException("from inner try");
}
} catch(TwoException e) {
System.out.println(
"Caught in outer try, e.printStackTrace()");
e.printStackTrace(System.out);
}
}
} /* Output:
originating the exception in f()
Caught in inner try, e.printStackTrace()
OneException: thrown from f()
at RethrowNew.f(RethrowNew.java:15)
at RethrowNew.main(RethrowNew.java:20)
Caught in outer try, e.printStackTrace()
TwoException: from inner try
at RethrowNew.main(RethrowNew.java:25)
*///:~
最后那个异常仅知道自己来自main()，而对f()一无所知。异常也是对象，所以也有垃圾回收机制回收。不用担心清理问题。
异常链：在捕获一个异常后抛出另一个异常，并且希望把原始异常的信息保存下来，这被称为异常链。
现在所有Throwable的子类在构造器中都可以接受一个cause对象作为参数，这个参数就用来表示原始异常，这样就可以把原始异常传递给新的异常。
有三种基本的异常类提供了带参数Error,Exception,RuntimeException:
//: exceptions/DynamicFields.java
// A Class that dynamically adds fields to itself.
// Demonstrates exception chaining.
import static net.mindview.util.Print.*;
class DynamicFieldsException extends Exception {}
public class DynamicFields {
private Object[][] fields;
public DynamicFields(int initialSize) {
fields = new Object[initialSize][2];
for(int i = 0; i < initialSize; i++)
fields[i] = new Object[] { null, null };
}
public String toString() {
StringBuilder result = new StringBuilder();
for(Object[] obj : fields) {
result.append(obj[0]);
result.append(": ");
result.append(obj[1]);
result.append("\n");
}
return result.toString();
}
private int hasField(String id) {
for(int i = 0; i < fields.length; i++)
if(id.equals(fields[i][0]))
return i;
return -1;
}
private int
getFieldNumber(String id) throws NoSuchFieldException {
int fieldNum = hasField(id);
if(fieldNum == -1)
throw new NoSuchFieldException();
return fieldNum;
}
private int makeField(String id) {
for(int i = 0; i < fields.length; i++)
if(fields[i][0] == null) {
fields[i][0] = id;
return i;
}
// No empty fields. Add one:
Object[][] tmp = new Object[fields.length + 1][2];
for(int i = 0; i < fields.length; i++)
tmp[i] = fields[i];
for(int i = fields.length; i < tmp.length; i++)
tmp[i] = new Object[] { null, null };
fields = tmp;
// Recursive call with expanded fields:
return makeField(id);
}
public Object
getField(String id) throws NoSuchFieldException {
return fields[getFieldNumber(id)][1];
}
public Object setField(String id, Object value)
throws DynamicFieldsException {
if(value == null) {
// Most exceptions don't have a "cause" constructor.
// In these cases you must use initCause(),
// available in all Throwable subclasses.
DynamicFieldsException dfe =
new DynamicFieldsException();
dfe.initCause(new NullPointerException());
throw dfe;
}
int fieldNumber = hasField(id);
if(fieldNumber == -1)
fieldNumber = makeField(id);
Object result = null;
try {
result = getField(id); // Get old value
} catch(NoSuchFieldException e) {
// Use constructor that takes "cause":
throw new RuntimeException(e);
}
fields[fieldNumber][1] = value;
return result;
}
public static void main(String[] args) {
DynamicFields df = new DynamicFields(3);
print(df);
try {
df.setField("d", "A value for d");
df.setField("number", 47);
df.setField("number2", 48);
print(df);
df.setField("d", "A new value for d");
df.setField("number3", 11);
print("df: " + df);
print("df.getField(\"d\") : " + df.getField("d"));
Object field = df.setField("d", null); // Exception
} catch(NoSuchFieldException e) {
e.printStackTrace(System.out);
} catch(DynamicFieldsException e) {
e.printStackTrace(System.out);
}
}
} /* Output:
null: null
null: null
null: null
d: A value for d
number: 47
number2: 48
df: d: A new value for d
number: 47
number2: 48
number3: 11
df.getField("d") : A new value for d
DynamicFieldsException
at DynamicFields.setField(DynamicFields.java:64)
at DynamicFields.main(DynamicFields.java:94)
Caused by: java.lang.NullPointerException
at DynamicFields.setField(DynamicFields.java:66)
... 1 more
*///:~
上例中使用了initCause()方式。
12.7Java标准异常
Throwable这个java类被用来表示任何可以作为异常被抛出的类。Throwable对象可以分为两种类型（继承Throwable）：Error用来表示编译时和系统错误；Exception时可以被抛出的基本类型，在Java类库，用户方法以及运行时故障中都可能抛出Exception异常。基本只需关注Exception即可。
异常的基本概念是用名称代表发生的事情。
//: exceptions/NeverCaught.java
// Ignoring RuntimeExceptions.
// {ThrowsException}
public class NeverCaught {
static void f() {
throw new RuntimeException("From f()");
}
static void g() {
f();
}
public static void main(String[] args) {
g();
}
} ///:~
只能在代码中忽略RuntimeException（子类）类型的异常。其他异常类型的处理都是由编译器强制执行的，RuntimeException代表的是编程错误：
无法预料的错误，比如null
一个地方的异常，导致另一个地方的错误。
不应该把Java异常处理机制当成是单一用途的工具，设计目的是用来处理运行时错误。
12.8使用finally进行清理
对于无论try块中是否抛出异常，都能得到执行，需要放到finally中：
//: exceptions/FinallyWorks.java
// The finally clause is always executed.
class ThreeException extends Exception {}
public class FinallyWorks {
static int count = 0;
public static void main(String[] args) {
while(true) {
try {
// Post-increment is zero first time:
if(count++ == 0)
throw new ThreeException();
System.out.println("No exception");
} catch(ThreeException e) {
System.out.println("ThreeException");
} finally {
System.out.println("In finally clause");
if(count == 2) break; // out of "while"
}
}
}
} /* Output:
ThreeException
In finally clause
No exception
In finally clause
*///:~
finally子句总会得到执行。
finally的作用：
//: exceptions/Switch.java
import static net.mindview.util.Print.*;
public class Switch {
private boolean state = false;
public boolean read() { return state; }
public void on() { state = true; print(this); }
public void off() { state = false; print(this); }
public String toString() { return state ? "on" : "off"; }
} ///:~
//: exceptions/OnOffException1.java
public class OnOffException1 extends Exception {} ///:~
//: exceptions/OnOffException2.java
public class OnOffException2 extends Exception {} ///:~
//: exceptions/OnOffSwitch.java
// Why use finally?
public class OnOffSwitch {
private static Switch sw = new Switch();
public static void f()
throws OnOffException1,OnOffException2 {}
public static void main(String[] args) {
try {
sw.on();
// Code that can throw exceptions...
f();
sw.off();
} catch(OnOffException1 e) {
System.out.println("OnOffException1");
sw.off();
} catch(OnOffException2 e) {
System.out.println("OnOffException2");
sw.off();
}
}
} /* Output:
on
off
*///:~
以上可用finally:
//: exceptions/WithFinally.java
// Finally Guarantees cleanup.
public class WithFinally {
static Switch sw = new Switch();
public static void main(String[] args) {
try {
sw.on();
// Code that can throw exceptions...
OnOffSwitch.f();
} catch(OnOffException1 e) {
System.out.println("OnOffException1");
} catch(OnOffException2 e) {
System.out.println("OnOffException2");
} finally {
sw.off();
}
}
} /* Output:
on
off
*///:~
总是会得到执行：
//: exceptions/AlwaysFinally.java
// Finally is always executed.
import static net.mindview.util.Print.*;
class FourException extends Exception {}
public class AlwaysFinally {
public static void main(String[] args) {
print("Entering first try block");
try {
print("Entering second try block");
try {
throw new FourException();
} finally {
print("finally in 2nd try block");
}
} catch(FourException e) {
System.out.println(
"Caught FourException in 1st try block");
} finally {
System.out.println("finally in 1st try block");
}
}
} /* Output:
Entering first try block
Entering second try block
finally in 2nd try block
Caught FourException in 1st try block
finally in 1st try block
*///:~
包括return语句之后也会执行：
//: exceptions/MultipleReturns.java
import static net.mindview.util.Print.*;
public class MultipleReturns {
public static void f(int i) {
print("Initialization that requires cleanup");
try {
print("Point 1");
if(i == 1) return;
print("Point 2");
if(i == 2) return;
print("Point 3");
if(i == 3) return;
print("End");
return;
} finally {
print("Performing cleanup");
}
}
public static void main(String[] args) {
for(int i = 1; i <= 4; i++)
f(i);
}
} /* Output:
Initialization that requires cleanup
Point 1
Performing cleanup
Initialization that requires cleanup
Point 1
Point 2
Performing cleanup
Initialization that requires cleanup
Point 1
Point 2
Point 3
Performing cleanup
Initialization that requires cleanup
Point 1
Point 2
Point 3
End
Performing cleanup
*///:~
不当使用会丢失异常：
//: exceptions/LostMessage.java
// How an exception can be lost.
class VeryImportantException extends Exception {
public String toString() {
return "A very important exception!";
}
}
class HoHumException extends Exception {
public String toString() {
return "A trivial exception";
}
}
public class LostMessage {
void f() throws VeryImportantException {
throw new VeryImportantException();
}
void dispose() throws HoHumException {
throw new HoHumException();
}
public static void main(String[] args) {
try {
LostMessage lm = new LostMessage();
try {
lm.f();
} finally {
lm.dispose();
}
} catch(Exception e) {
System.out.println(e);
}
}
} /* Output:
A trivial exception
*///:~
//: exceptions/ExceptionSilencer.java
public class ExceptionSilencer {
public static void main(String[] args) {
try {
throw new RuntimeException();
} finally {
// Using 'return' inside the finally block
// will silence any thrown exception.
return;
}
}
} ///:~
12.9异常的限制
当覆盖方法的时候，只能抛出在某类方法的异常说明里列出的那些异常。面向对象的概念，在继承里同样适用：
//: exceptions/StormyInning.java
// Overridden methods may throw only the exceptions
// specified in their base-class versions, or exceptions
// derived from the base-class exceptions.
class BaseballException extends Exception {}
class Foul extends BaseballException {}
class Strike extends BaseballException {}
abstract class Inning {
public Inning() throws BaseballException {}
public void event() throws BaseballException {
// Doesn't actually have to throw anything
}
public abstract void atBat() throws Strike, Foul;
public void walk() {} // Throws no checked exceptions
}
class StormException extends Exception {}
class RainedOut extends StormException {}
class PopFoul extends Foul {}
interface Storm {
public void event() throws RainedOut;
public void rainHard() throws RainedOut;
}
public class StormyInning extends Inning implements Storm {
// OK to add new exceptions for constructors, but you
// must deal with the base constructor exceptions:
public StormyInning()
throws RainedOut, BaseballException {}
public StormyInning(String s)
throws Foul, BaseballException {}
// Regular methods must conform to base class:
//! void walk() throws PopFoul {} //Compile error
// Interface CANNOT add exceptions to existing
// methods from the base class:
//! public void event() throws RainedOut {}
// If the method doesn't already exist in the
// base class, the exception is OK:
public void rainHard() throws RainedOut {}
// You can choose to not throw any exceptions,
// even if the base version does:
public void event() {}
// Overridden methods can throw inherited exceptions:
public void atBat() throws PopFoul {}
public static void main(String[] args) {
try {
StormyInning si = new StormyInning();
si.atBat();
} catch(PopFoul e) {
System.out.println("Pop foul");
} catch(RainedOut e) {
System.out.println("Rained out");
} catch(BaseballException e) {
System.out.println("Generic baseball exception");
}
// Strike not thrown in derived version.
try {
// What happens if you upcast?
Inning i = new StormyInning();
i.atBat();
// You must catch the exceptions from the
// base-class version of the method:
} catch(Strike e) {
System.out.println("Strike");
} catch(Foul e) {
System.out.println("Foul");
} catch(RainedOut e) {
System.out.println("Rained out");
} catch(BaseballException e) {
System.out.println("Generic baseball exception");
}
}
} ///:~
在Inning类中，可以看到构造器和event()方法都声明将抛出异常，但实际没有抛出。这种方式能够强制用户捕获可能在覆盖后的event()版本中增加的异常。
异常限制对构造器不起作用。
派生类构造器不能捕获基类构造器捕获的异常。
一个出现在基类方法的异常说明中的异常，不一定会出现在派生类方法的异常说明里。
12.10构造器
//: exceptions/InputFile.java
// Paying attention to exceptions in constructors.
import java.io.*;
public class InputFile {
private BufferedReader in;
public InputFile(String fname) throws Exception {
try {
in = new BufferedReader(new FileReader(fname));
// Other code that might throw exceptions
} catch(FileNotFoundException e) {
System.out.println("Could not open " + fname);
// Wasn't open, so don't close it
throw e;
} catch(Exception e) {
// All other exceptions must close it
try {
in.close();
} catch(IOException e2) {
System.out.println("in.close() unsuccessful");
}
throw e; // Rethrow
} finally {
// Don't close it here!!!
}
}
public String getLine() {
String s;
try {
s = in.readLine();
} catch(IOException e) {
throw new RuntimeException("readLine() failed");
}
return s;
}
public void dispose() {
try {
in.close();
System.out.println("dispose() successful");
} catch(IOException e2) {
throw new RuntimeException("in.close() failed");
}
}
} ///:~
catch{}中再加try finally
//: exceptions/Cleanup.java
// Guaranteeing proper cleanup of a resource.
public class Cleanup {
public static void main(String[] args) {
try {
InputFile in = new InputFile("Cleanup.java");
try {
String s;
int i = 1;
while((s = in.getLine()) != null)
; // Perform line-by-line processing here...
} catch(Exception e) {
System.out.println("Caught Exception in main");
e.printStackTrace(System.out);
} finally {
in.dispose();
}
} catch(Exception e) {
System.out.println("InputFile construction failed");
}
}
} /* Output:
dispose() successful
*///:~
finally再构造失败不会执行，成功的时候会执行。
再创建一个需要清理的语句之后马上进入try finally:
//: exceptions/CleanupIdiom.java
// Each disposable object must be followed by a try-finally
class NeedsCleanup { // Construction can't fail
private static long counter = 1;
private final long id = counter++;
public void dispose() {
System.out.println("NeedsCleanup " + id + " disposed");
}
}
class ConstructionException extends Exception {}
class NeedsCleanup2 extends NeedsCleanup {
// Construction can fail:
public NeedsCleanup2() throws ConstructionException {}
}
public class CleanupIdiom {
public static void main(String[] args) {
// Section 1:
NeedsCleanup nc1 = new NeedsCleanup();
try {
// ...
} finally {
nc1.dispose();
}
// Section 2:
// If construction cannot fail you can group objects:
NeedsCleanup nc2 = new NeedsCleanup();
NeedsCleanup nc3 = new NeedsCleanup();
try {
// ...
} finally {
nc3.dispose(); // Reverse order of construction
nc2.dispose();
}
// Section 3:
// If construction can fail you must guard each one:
try {
NeedsCleanup2 nc4 = new NeedsCleanup2();
try {
NeedsCleanup2 nc5 = new NeedsCleanup2();
try {
// ...
} finally {
nc5.dispose();
}
} catch(ConstructionException e) { // nc5 constructor
System.out.println(e);
} finally {
nc4.dispose();
}
} catch(ConstructionException e) { // nc4 constructor
System.out.println(e);
}
}
} /* Output:
NeedsCleanup 1 disposed
NeedsCleanup 3 disposed
NeedsCleanup 2 disposed
NeedsCleanup 5 disposed
NeedsCleanup 4 disposed
*///:~
12.11异常匹配
抛出异常的时候，异常处理系统会按照代码的书写顺序找出“最近”的处理程序。找到匹配的处理程序后，就认为异常得到处理，然后不再继续查找。派生类可以匹配基类的处理程序：
//: exceptions/Human.java
// Catching exception hierarchies.
class Annoyance extends Exception {}
class Sneeze extends Annoyance {}
public class Human {
public static void main(String[] args) {
// Catch the exact type:
try {
throw new Sneeze();
} catch(Sneeze s) {
System.out.println("Caught Sneeze");
} catch(Annoyance a) {
System.out.println("Caught Annoyance");
}
// Catch the base type:
try {
throw new Sneeze();
} catch(Annoyance a) {
System.out.println("Caught Annoyance");
}
}
} /* Output:
Caught Sneeze
Caught Annoyance
*///:~
12.12其他可选方式
只有在你知道如何处理的情况下才捕获。
12.13异常使用指南
应该在下列情况使用异常：
在恰当的级别处理问题（在知道如何处理情况下才捕获异常）
解决问题，并且重新调用产生异常的方法
进行少许修补，然后绕过异常发生的地方继续执行
用别的数据进行计算，以代替方法预计返回的值
把当前环境下能做的事尽量做完，然后把相同的异常抛到更高层，把不同的异常抛到更高层
终止程序
进行简化
让类库程序更安全
12.14总结
异常处理的优点就是它使得你可以在某处集中精力处理你要解决的问题，而在另一处处理你编写的这段代码产生的问题。
