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The finally clause closes the reader object in this noncompliant example. However, it is incorrectly assumed that the statements within the finally block cannot throw exceptions. Notably, close() can throw an IOException which in turn prevents any subsequent clean-up lines from getting executed. This is not detected at compile time since close() throws the same exception type as read or write.
| Code Block | ||
|---|---|---|
| ||
import java.io.IOException;
import java.io.BufferedReader;
import java.io.FileReader;
public class Login {
static void checkPassword(String password_file) throws IOException {
StringBuffer fileData = new StringBuffer(1000);
BufferedReader reader = new BufferedReader(new FileReader(password_file));
try {
int n;
char[] passwd = new char[1024];
while ((n = reader.read(passwd)) >= 0) {
String readData = String.valueOf(passwd, 0, n);
fileData.append(readData);
passwd = new char[1024];
}
String realPassword = "javac<at:var at:name="f3b" />b3";
System.out.println(fileData.toString());
if (fileData.toString().equals(realPassword)) {
System.out.println("Login successful");
}
else {
System.out.println("Login failed");
}
} finally {
reader.close();
//other clean-up code
}
}
public static void main(String[] args) throws IOException {
String path = "c:\\password.txt";
checkPassword(path);
}
}
|
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This compliant solution correctly places the close() statement in a try-catch block. As a result an IOException can be handled without letting it propagate any further.
| Code Block | ||
|---|---|---|
| ||
import java.io.IOException;
import java.io.BufferedReader;
import java.io.FileReader;
public class Login {
static void checkPassword(String password_file) throws IOException {
StringBuffer fileData = new StringBuffer(1000);
BufferedReader reader = new BufferedReader(new FileReader(password_file));
try {
int n;
char[] passwd = new char[1024];
while ((n = reader.read(passwd)) >= 0) {
String readData = String.valueOf(passwd, 0, n);
fileData.append(readData);
passwd = new char[1024];
}
String realPassword = "javac<at:var at:name="f3b" />b3";
System.out.println(fileData.toString());
if (fileData.toString().equals(realPassword)) {
System.out.println("Login successful");
}
else {
System.out.println("Login failed");
}
} finally {
try { //enclose in try-catch block
reader.close();
//other clean-up code
}catch (IOException ie) {ie.getMessage();}
}
}
public static void main(String[] args) throws IOException {
String path = "c:\\password.txt";
checkPassword(path);
}
}
|
...
If the need to close a stream without throwing an exception occurs often, then an alternative solution to wrapping every call of close() in its own try-catch block is to write a method, as shown in this compliant solution.
| Code Block | ||
|---|---|---|
| ||
import java.io.IOException;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.Closeable;
public class Login {
static void checkPassword(String password_file) throws IOException {
StringBuffer fileData = new StringBuffer(1000);
BufferedReader reader = new BufferedReader(new FileReader(password_file));
try {
int n;
char[] passwd = new char[1024];
while ((n = reader.read(passwd)) >= 0) {
String readData = String.valueOf(passwd, 0, n);
fileData.append(readData);
passwd = new char[1024];
}
String realPassword = "javac<at:var at:name="f3b" />b3";
System.out.println(fileData.toString());
if (fileData.toString().equals(realPassword)) {
System.out.println("Login successful");
}
else {
System.out.println("Login failed");
}
} finally {
closeIgnoringException(reader);
//other clean-up code
}
}
private static void closeIgnoringException(Closeable s) {
if (s != null) {
try {
s.close();
} catch (IOException ie) {
// Ignore exception if close fails
}
}
}
public static void main(String[] args) throws IOException {
String path = "c:\\password.txt";
checkPassword(path);
}
}
|
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| Wiki Markup |
|---|
\[[Bloch 05|AA. Java References#Bloch 05]\] Puzzle 41: Field and Stream
\[[Harold 99|AA. Java References#Harold 99]\]
\[[Chess 07|AA. Java References#Chess 07]\] 8.3 Preventing Resource Leaks (Java) |
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EXC30-J. Do not exit abruptly from a finally block 10. Exceptional Behavior (EXC) 11. Miscellaneous (MSC)