这是三个问题,但这不是第一次发生在这里。 :-)
是的,你必须处理它们或声明它们,因为它们是检查异常。你必须这么做的原因是,调用你的代码的代码知道你的代码可能失败的方式(因为你声明了它可以抛出的异常)。
这段代码非常简单,所以分离和收益并不是那么明显。但考虑打开两个流,使用转换代码(包括调用从属方法)从一个到另一个进行复制。最终你会得到一个有10-20个语句的方法体。而不是每个I/O语句必须检查它是否工作,只需编写包含在IOException
处理程序中的逻辑,知道任何I/O异常都会跳出主逻辑处理程序。
这取决于您正在编写什么类型的程序,但通常情况下,您会在最适合的级别处理异常,这通常是在程序中的多个级别。最外层只处理真正非常不寻常的不可恢复的异常,或者只是让缺省处理完成它所做的事情,或者使用一个完全相同的处理程序,但可能会(尝试)记录失败在其他地方(如日志文件),以及:
public class MyProgram {
public static final void main(String[] args) {
try {
// Run...
}
catch (Throwable t) {
// Handle the fact that something went wrong here, if you can
// Usually this would be only for really, really unusual errors,
// otherwise you would have handled them earlier
}
}
}
为了强调在#2点,考虑两种process
方法,一个在Java中有例外,和其他在一个假设的类似Java的语言没有例外:
Java之一:
private void process() {
try ( // <== Main logic
Reader fr = new FileReader(this.sourceFileName); // <== Main logic
BufferedReader br = new BufferedReader(fr); // <== Main logic
Writer fw = new FileWriter(this.destFileName); // <== Main logic
BufferedWriter bw = new BufferedWriter(fw) // <== Main logic
) { // <== Main logic
String line; // <== Main logic
while ((line = br.readLine()) != null) { // <== Main logic
if (shouldIncludeLine(line)) { // <== Main logic
line = transformLine(line); // <== Main logic
bw.write(line); // <== Main logic
bw.newLine(); // <== Main logic
} // <== Main logic
} // <== Main logic
}
catch (FileNotFoundException fnfe) { // <== Error handling
// Couldn't find a file // <== Error handling
// (handle it) // <== Error handling
} // <== Error handling
catch (IOException ioe) { // <== Error handling
// I/O error // <== Error handling
// (handle it) // <== Error handling
} // <== Error handling
catch (Exception e) { // <== Error handling
// Something else went wrong // <== Error handling
// (handle it) // <== Error handling
} // <== Error handling
}
假设的类似Java的语言没有例外之一:
// THIS IS FAKE, PSEUDO-JAVA
private Errors process() {
Reader fr = new FileReader(this.sourceFileName); // <== Main logic
if (fr == null) { // <== Error handling
return Errors.CantOpenSource; // <== Error handling
} // <== Error handling
BufferedReader br = new BufferedReader(fr); // <== Main logic
Writer fw = new FileWriter(this.destFileName); // <== Main logic
if (fw == null) { // <== Error handling
br.close(); // <== Error handling
return Errors.CantOpenDest; // <== Error handling
} // <== Error handling
BufferedWriter bw = new BufferedWriter(fw) // <== Main logic
String line; // <== Main logic
while ((line = br.readLine()) != IO.END_OF_FILE) { // <== Main logic
if (line == null) { // <== Error handling
br.close(); // <== Error handling
bw.close(); // <== Error handling
return Errors.CantRead; // <== Error handling
}
if (shouldIncludeLine(line)) { // <== Main logic
line = transformLine(line); // <== Main logic
if (bw.write(line) == -1 || bw.newLine() == -1) { // <== Main logic (plus some error handling)
br.close(); // <== Error handling
bw.close(); // <== Error handling
return Errors.CantWrite; // <== Error handling
}
}
}
bw.close();
br.close();
return Errors.Success;
}
注意:
- 的主要逻辑是如何充斥着错误处理,使其难以阅读和遵守。
- 特殊的“错误”返回值对于任何可能存在某种故障模式的方法都是必需的。我们必须添加一个
process
,然后我们从new FileReader
和这样的检查null
,和读取检查-1和写OPS等
如果你有兴趣,这里是一个完整的Java程序的版本与完整版的没有,真正的Java程序:
的Java:
import java.io.*;
public class Example
{
private String sourceFileName;
private String destFileName;
public static void main (String[] args) throws java.lang.Exception
{
try {
new Example(args[0], args[1]).process();
}
catch (ArrayIndexOutOfBoundsException npe) {
// This is a bit of an exaggeration, I'd check in advance, since the user not
// supplying arguments isn't really an "exceptional" condition.
System.out.println("Usage: java Example [source file name] [dest file name]");
}
}
public Example(String src, String dest) {
// Similar, these checks would probably be assertions, but I'm making a point...
if (src == null || src.length() == 0) {
throw new IllegalArgumentException("src must be non-null and non-blank");
}
if (dest == null || dest.length() == 0) {
throw new IllegalArgumentException("dest must be non-null and non-blank");
}
this.sourceFileName = src;
this.destFileName = dest;
}
private void process() {
try ( // <== Main logic
Reader fr = new FileReader(this.sourceFileName); // <== Main logic
BufferedReader br = new BufferedReader(fr); // <== Main logic
Writer fw = new FileWriter(this.destFileName); // <== Main logic
BufferedWriter bw = new BufferedWriter(fw) // <== Main logic
) { // <== Main logic
String line; // <== Main logic
while ((line = br.readLine()) != null) { // <== Main logic
if (shouldIncludeLine(line)) { // <== Main logic
line = transformLine(line); // <== Main logic
bw.write(line); // <== Main logic
bw.newLine(); // <== Main logic
} // <== Main logic
} // <== Main logic
}
catch (FileNotFoundException fnfe) { // <== Error handling
// Couldn't find a file // <== Error handling
// (handle it) // <== Error handling
} // <== Error handling
catch (IOException ioe) { // <== Error handling
// I/O error // <== Error handling
// (handle it) // <== Error handling
} // <== Error handling
catch (Exception e) { // <== Error handling
// Something else went wrong // <== Error handling
// (handle it) // <== Error handling
} // <== Error handling
}
private boolean shouldIncludeLine(String line) {
return line.length() != 0;
}
private String transformLine(String line) {
return line.toUpperCase();
}
}
毫无例外一个假设的类似Java的语言:
// THIS IS FAKE, PSEUDO-JAVA WITHOUT EXCEPTIONS, IT ISN'T REAL
import java.io.*;
public class Example
{
private String sourceFileName;
private String destFileName;
private enum Errors {
Success,
CantOpenSource,
CantOpenDest,
CantRead,
CantWrite
}
public static void main (String[] args) throws java.lang.Exception
{
if (args.length < 2) {
System.out.println("Usage: java Example [source file name] [dest file name]");
}
if (args[0] == null || args[0].length() == 0) {
throw new IllegalArgumentException("src must be non-null and non-blank");
}
if (args[1] == null || args[1].length() == 0) {
throw new IllegalArgumentException("dest must be non-null and non-blank");
}
switch (new Example(args[0], args[1]).process()) {
case Errors.CantOpenSource:
// Handle it
break;
case Errors.CantOpenDest:
// Handle it
break;
case Errors.CantRead:
// Handle it
break;
case Errors.CantWrite:
// Handle it
break;
}
}
public Example(String src, String dest) {
// Not how now this constructor is trusting that it is called with valid arguments
this.sourceFileName = src;
this.destFileName = dest;
}
private Errors process() {
Reader fr = new FileReader(this.sourceFileName); // <== Main logic
if (fr == null) { // <== Error handling
return Errors.CantOpenSource; // <== Error handling
} // <== Error handling
BufferedReader br = new BufferedReader(fr); // <== Main logic
Writer fw = new FileWriter(this.destFileName); // <== Main logic
if (fw == null) { // <== Error handling
br.close(); // <== Error handling
return Errors.CantOpenDest; // <== Error handling
} // <== Error handling
BufferedWriter bw = new BufferedWriter(fw) // <== Main logic
String line; // <== Main logic
while ((line = br.readLine()) != IO.END_OF_FILE) { // <== Main logic
if (line == null) { // <== Error handling
br.close(); // <== Error handling
bw.close(); // <== Error handling
return Errors.CantRead; // <== Error handling
}
if (shouldIncludeLine(line)) { // <== Main logic
line = transformLine(line); // <== Main logic
if (bw.write(line) == -1 || bw.newLine() == -1) { // <== Main logic (plus some error handling)
br.close(); // <== Error handling
bw.close(); // <== Error handling
return Errors.CantWrite; // <== Error handling
}
}
}
bw.close();
br.close();
return Errors.Success;
}
private boolean shouldIncludeLine(String line) {
return line.length() != 0;
}
private String transformLine(String line) {
return line.toUpperCase();
}
}
这是三个不同的问题,而不是* a *问题。 –
#1中的问题是什么?是的,你必须处理它们或宣布它们。 –
关于Q2,假设'result'被用作业务逻辑中的一个关键值,那么只有当你的代码是一个有效变量的时候,你才想把'result'赋给你的代码,在'try'块中如果没有抛出异常,则使用'result'。如果抛出异常,你可以在'catch'块中给'result'一个默认值。因此,您的业务逻辑代码从不受由于result变量引发的异常的影响。 – OPK