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Native methods are defined in Java and written in traditional languages such as C/C++ (see \[[JNI 06|AA. Java References#JNI 06]\]). The added extensibility comes at the cost of flexibility and portability as the code no longer conforms to the policies enforced by Java. In the past, native method were used for performing platform specific operations, interfacing with legacy library code and improving program performance \[[Bloch 08|AA. Java References#Bloch 08]\]. Although this is not completely true in present times (due to poor portability, safety and quite ironically, performance issues), they are still used to interface with legacy code. |
Defining a wrapper method facilitates carrying out security manager checks, perform input validation before passing the arguments to the native code, defensively copy mutable inputs and sanitize user supplied input.
Noncompliant Code Example
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public final class NativeMethod {
// public native method
public native void nativeOperation(byte[] data, int offset, int len);
// wrapper method that does not perform any security checks or input validation
public void doOperation(byte[] data, int offset, int len) {
nativeOperation(data, offset, len);
}
static {
System.loadLibrary("NativeMethodLib"); //load native library in static initializer of class
}
}
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This compliant solution makes the actual native method private and defines a public wrapper that calls securityManagerCheck() which in turn performs routine permission checks to determine if the succeeding operations can continue. This is followed by input range checking and creation of a copy of the mutable input array, data. Finally the nativeOperation method is called with sanitized inputs. Ensure that the validation checks produce outputs that are coherent with the input requirements of the native implementations/libraries.
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public final class NativeMethodWrapper {
// private native method
private native void nativeOperation(byte[] data, int offset, int len);
// wrapper method performs SecurityManager and input validation checks
public void doOperation(byte[] data, int offset, int len) {
// permission needed to invoke native method
securityManagerCheck();
if (data == null) {
throw new NullPointerException();
}
// copy mutable input
data = data.clone();
// validate input
if ((offset < 0) || (len < 0) || (offset > (data.length - len))) {
throw new IllegalArgumentException();
}
nativeOperation(data, offset, len);
}
static {
System.loadLibrary("NativeMethodLib"); //load native library in static initializer of class
}
}
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