Errors and Exceptions in Java

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Example 84 This Java code fails to check the return value of read(), making it possible for private data to leak between users
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FileInputStream fis; byte[] byteArray = new byte[1024]; for (Iterator i=usersiterator(); ihasNext();) { String userName = (String) inext(); String pFileName = PFILE_ROOT + "/" + userName; FileInputStream fis = new FileInputStream(pFileName); try { fisread(byteArray); // the file is always 1k bytes processPFile(userName, byteArray); } finally { fisclose(); } }
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Example 85 shows the fix for this problem Now the program continues to call read() until the expected amount of data arrives If the expected amount of data is not present, it throws an exception
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Example 85 Checking the return value from read() xes the bug but makes the code more complex
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FileInputStream fis; byte[] byteArray = new byte[1024]; for (Iterator i=usersiterator(); ihasNext();) { String userName = (String) inext(); String pFileName = PFILE_ROOT + "/" + userName; fis = new FileInputStream(pFileName); try { int bRead = 0; while (bRead < 1024) { int rd = fisread(byteArray, bRead, 1024 - bRead); if (rd == -1) { throw new IOException("file is unusually small"); } bRead += rd; } } finally { fisclose(); } // could add check to see if file is too large here processPFile(userName, byteArray) ; }
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Because the fix for this problem is relatively complicated (it requires more than half a dozen new lines of code, including the addition of a loop), you might be tempted to look for a simpler answer, such as checking the size of the file before you begin reading Such an approach would render the application vulnerable to a file access race condition, whereby an attacker could replace a well-formed file with a malicious file in the window of time between the file size check and the call to read data from the file
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Exceptions solve many error handling problems Although it is easy to ignore a function s return value by simply omitting the code to look at it, ignoring a checked exception requires just the opposite a programmer has to write code speci cally to ignore it Exceptions also allow for separation between code that follows an expected path and code that handles abnormal circumstances Exceptions come in two avors: checked and unchecked The difference has to do with whether the compiler will use static analysis to ensure that the exception is handled If a method declares that it throws a checked exception, all methods that call it must either handle the exception or declare that they throw it as well This forces the programmer to think about checked exceptions wherever they might occur Java compilers enforce the rules regarding checked exceptions, and the Java class library makes liberal use of checked exceptions The Java class javalangException is a checked exception Unchecked exceptions do not have to be declared or handled All exceptions in C++ are unchecked, which means that a programmer could completely ignore the fact that exceptions are possible and the compiler would not complain Java offers unchecked exceptions, too The danger with unchecked exceptions is that programmers might be unaware that an exception can occur in a given context and might omit appropriate error handling For example, the Windows _alloca() function allocates memory on the stack If an allocation request is too large for the available stack space, _alloca() throws an unchecked stack overflow exception If the exception is not caught, the program will crash,
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8 Errors and Exceptions
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potentially enabling a denial-of-service attack2 The Windows function EnterCriticalSection() can raise an exception in low-memory situations, which can cause similar problems Because there is no compile-time indication that an exception might occur, programmers can easily forget to account for the possibility Catch Everything at the Top Level To shut down gracefully and avoid leaking a stack trace or other system information, programs should declare a safety-net exception handler that deals with any exceptions (checked or unchecked) that percolate to the top of the call stack Example 86 shows the doPost() method for a Java Servlet If a DNS lookup failure occurs, the Servlet will throw an UnknownHostException Also, depending on the operations it performs, the proprietary UtilsprocessHost() method might throw other exceptions Because these exceptions go uncaught, the code implicitly relies on the Servlet container s configuration to prevent users from seeing a stack trace or other system-debugging information that could be useful in mounting an attack For examples of bad error pages we ve seen, refer to the Error Handling section in 9, Web Applications
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