OVERVIEW OF THE JNI DESIGN in Java

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OVERVIEW OF THE JNI DESIGN
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not be overloaded in the Java programming language It is legal, however, to have overloaded elds in a class le, and to run such class les on Java virtual machines Therefore, the JNI is able to handle legal class les that are not generated by a compiler for the Java programming language Programmers can use the JNI to call methods or access elds only if they already know the names and types of the methods or elds In comparison, the Java Core Re ection API allows programmers to determine the set of elds and methods in a given class or interface It is sometimes useful to be able to re ect on class or interface types in native code as well Java 2 SDK release 12 provides new JNI functions that are designed to work with the existing Java Core Re ection API The new functions include one pair that converts between JNI eld IDs and instances of the javalangreflectField class, and another pair that converts between JNI method IDs and instances of the javalangreflectMethod class
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118 Errors and Exceptions
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Errors made in JNI programming are different from exceptions that occur in the Java virtual machine implementation Programmer errors are caused by misuses of JNI functions The programmer, for example, may mistakenly pass an object reference instead of a class reference to GetFieldID Java virtual machine exceptions are raised, for example, by out-of-memory situations that occur when native code tries to allocate an object through the JNI 1181 No Checking for Programming Errors The JNI functions do not check for programming errors Passing illegal arguments to JNI functions results in unde ned behavior The reason for this design decision is as follows: Forcing JNI functions to check for all possible error conditions degrades the performance in all (typically correct) native methods In many cases there is not enough runtime type information to perform such checking Most C library functions do not guard against programming errors The
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printf function, for example, usually triggers a runtime error instead of returning
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an error code when it receives an invalid address Forcing C library functions to check for all possible error conditions would likely result in such checks being duplicated, once in the user code and then again in the library 161
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Java Virtual Machine Exceptions
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OVERVIEW OF THE JNI DESIGN
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Although the JNI speci cation does not require the virtual machine to check for programming errors, virtual machine implementations are encouraged to provide checks for common mistakes For example, a virtual machine may perform more checking in a debug version of the JNI function table ( 1152) 1182 Java Virtual Machine Exceptions The JNI does not rely on exception handling mechanisms in native programming languages Native code may cause the Java virtual machine to throw an exception by calling Throw or ThrowNew A pending exception is recorded in the current thread Unlike exceptions thrown in the Java programming language, exceptions thrown in native code do not immediately disrupt the current execution There is no standard exception handling mechanism in native languages Thus, JNI programmers are expected to check for and handle exceptions after each operation that can potentially throw an exception JNI programmers may deal with an exception in two ways: The native method may choose to return immediately, causing the exception to be thrown in the code that initiated the native method call The native code may clear the exception by calling ExceptionClear and then execute its own exception-handling code It is extremely important to check, handle, and clear a pending exception before calling any subsequent JNI functions Calling most JNI functions with a pending exception leads to unde ned results The following is the complete list of JNI functions that can be called safely when there is a pending exception:
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ExceptionOccurred ExceptionDescribe ExceptionClear ExceptionCheck ReleaseStringChars ReleaseStringUTFchars ReleaseStringCritical Release<Type>ArrayElements ReleasePrimitiveArrayCritical DeleteLocalRef DeleteGlobalRef DeleteWeakGlobalRef MonitorExit
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The rst four functions are directly related to exception handling The remaining ones are common in that they release various virtual machine resources 162
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