I am wondering is it possible to verify in Java under the Android SDK that a method in a Java class implemented as a native JNI method was resolved statically? Below there is an explanation of what I am looking for.
I have a Java class that is partially implemented as a JNI class. This class can be initialized statically if the corresponding JNI library has been created as a static library (libhelper.a, for instance). Or it can be initialized dynamically if the corresponding JNI library is implemented as a dynamic library (libhelper.so, for instance). In case of dynamic initialization the class should have a static initializer that loads the dynamic library – libhelper.so. I am using both case and I want to keep the same source code for both of them. For this purpose I would like to verify in the static initializer if the corresponding native methods has been already resolved. If it is true, I do not need to load dynamic library. If it is false, it means that I have to load dynamic library. The problem is I do not know how to verify that a method in the class has been already resolved.
The sample below has incorrect lines, that show my intention.
package com.sample.package;
public class MyUtilityClass
{
private static final String TAG = "MyUtilityClass";
public MyUtilityClass () {
Log.v(TAG, " MyUtilityClass constructor");
}
public static native int initMyHelperClass();
public static native int performHelpAction(String action);
public static native int uninitMyHelperClass();
static {
try {
/* Here I want to verify that the native method
initMyHelperClass has has been already resolved.
In this code snippet I am just comparing it to null,
which is not correct. It should be something different. */
if (initMyHelperClass == null) {
/* initMyHelperClass has not been resolved yet,
load the dynamic library - libhelper.so */
System.loadLibrary("helper");
}
} catch (UnsatisfiedLinkError ule) {
/*Library not found. We should throw second exception. */
throw ule;
}
}
}
Thank you.
You could use UnsatisfiedLinkError and a dummy method to check if a given class' native methods are loaded:
private static native void checkMe(); // does nothing
static {
try {
checkMe();
} catch (UnsatisfiedLinkError e) {
System.loadLibrary("checkLibrary");
}
}
Related
In normal Java class, when VM load a class, it will invoke clinit method, so I wonder know when VM load a interface, can it invoke some code?
for example, class B implements A, new B(), VM invoke clinit of B, what will VM do with A, in A can I insert some code like System.out.println("hello")
Directly not, Java interfaces are not supposed to contain any code, even if you can now have default method. Following code will not compile:
interface Foo {
init {
System.out.println("Loading Foo...");
}
}
However, interfaces can contain static fields:
interface Foo {
static class FooLoader {
private static Object init() {
System.out.printf("Initializing %s%n", Foo.class);
}
}
Object NULL = FooLoader.init();
}
Again, it may work BUT:
through Reflection, it's still possible to invoke init() method, so it can be called twice
code isn't really called at load time but at init time. To understand, what I mean check this simple main:
System.out.println("START");
System.out.println(Foo.class);
System.out.println("END");
As long as you don't access static members, Java interfaces are not initialized (See §5.5 of JVM Specification)
So, to truely catch load time, you can use a custom class loader, or instrumentation API.
Having static {} block in interfaces isn't possible. But if you are really certain that you need to invoke some code when loading interface you can use custom classloader which will hook your interface loading and perform some action on that
Here is an example:
static class MyClassLoader extends ClassLoader {
#Override
public Class<?> loadClass(String name) throws ClassNotFoundException {
if (name.equals("test.Test1")) {
... do whatewer you need on loading class/interface...
}
return getParent().loadClass(name);
}
}
}
How to replace classes in a running application in java ?
Also there is very usefull tutorial: https://zeroturnaround.com/rebellabs/reloading-objects-classes-classloaders/
As mentioned in another answers, you cannot have static section in interfaces. However you can have static methods and static final fields. You can combine both for debugging purposes.
interface TestInterface {
int dummy = init();
static int init() {
System.out.println("Loaded TestInterface");
return 1;
}
}
So I have two projects A and B, and project B is imported in project A, and in project B I want to initialize some objects which have static initializers.
The problem is, they aren't getting called (already tested with final keyword, does not help).
So I actually want to have a small system and it should go this way (every class decribed here are in project B):
class A is a main class in which you can call a method addClassToLoad()* to add other classes (which will be "loaded" when method start() in class A will be called);
classes B, C and D call method addClassToLoad() from its static initializer;
when some class from project A calls a method start(), class A lists all classes it has gotten and calls a method onLoad() (explained in *).
And every method is static, so it's meant to be only one (no "instancing").
Saddly, static initializers aren't getting called.
And the question is: do I do something wrong (or maybe it is not possible at all) or maybe there is another way to do this small system? (I just don't really want to write in class A about every class, which must be loaded at start() method)
*addClassToLoad() takes an interface which has one method onLoad(), so it is getting called when method start() is called in class A
In code version:
class A:
public class A {
private static ArrayList<ClassToLoad> classesToLoad;
public static void addClassToLoad(ClassToLoad c) {
if (classesToLoad == null)
classesToLoad = new ArrayList<ClassToLoad>();
classesToLoad.add(c);
}
public static void start() {
for (ClassToLoad c : classesToLoad) {
c.onLoad();
}
}
}
class B (and others (C, D etc.) like this one):
public class B {
static {
A.addClassToLoad(new ClassToLoad() {
public void onLoad() {
load();
}
});
}
private static void load() {
// do something here on load ...
}
}
class ClassToLoad:
public interface ClassToLoad {
public void onLoad();
}
This is the same question when you add a new JDBC driver, why you have to call Class.forName() to register a JDBC driver. Every JDBC driver class has a static initializer to register itself with DriverManager. Why? A class loader does not by default load all the classes in jar files. A class is loaded only when it is referenced during execution, which is smart as the class loader never has to load those unused classes into memory. So to resolve your issue, you have to manage to load those classes, like by Class.forName() before you call start(). If you use spring, you can create a list of all those classes in your configure. Spring also provides an util to scan packages for certain types of classes, then you can just specify a package name to scan.
Static fields will be set, and static initializers (static blocks) when ClassLoader will load class for the first time. Rembemer that this will happen when given class will be used for the first time as ClassLoader loads classes in lazy fashion (when needed)
So it seems it's not possible for me to execute those static blocks, so I added every class, which I need to load, into class A, and that way they're actually loading without any problems (in project B). And in project A I need to add other classes, which I need to load, in the main class, obviously.
So I made those classes as Singletons, so they're actually loaded and are ready for "main" loading, launching and disposing. So the adding class looks like this:
A.addClassToLoad(B.getInstance());
I used class ClassToLoad as a generic class to load (sounds funny), though I renamed it to SystemCycle.
So the code as an example of class B now looks like this:
public class B implements SystemCycle {
private static B instance = new B();
private B() {}
public static void getInstance() {
return instance;
}
public void onLoad() { /* some code here */ }
public void onLaunch() { /* some code here */ }
public void onDispose() { /* some code here */ }
}
And SystemCycle class looks now like this:
public interface SystemCycle {
public void onLoad();
public void onLaunch();
public void onDispose();
}
Well, that was obvious, because of example of class B.
And I even made small checking system, so if the user tries to call one of these methods, it will be ignored, as the class implementing SystemCycle checks whether the class A is actually loading, launching or disposing at that moment. But if not, it just can do return. (though if the usermade class doesn't check that, it can be abused by other usermade class).
P.S. addClassToLoad in my project is actually called addSystemToLoad, so I made it here this way to make an example easier to understand.
Small edit: I even tried something to do with annotations first, but even that thing didn't help me.
I'm trying to write an expression or series of statements of Java source code that when written inside a static method evaluates to null, but if the method is non-static evaluates to this.
My initial idea was to 'overload' on static vs non-static, as below:
public class test {
public void method1() {
System.out.println(getThisOrNull());
}
public static void method2() {
System.out.println(getThisOrNull());
}
private static Object getThisOrNull() {
return null;
}
private Object getThisOrNull() {
return this;
}
public static void main(String[] args) {
test t = new test();
System.out.println(t);
t.method1();
t.method2();
}
}
Unfortunately this isn't actually legal Java, you can't 'overload' like that and it just gives a compiler error:
test.java:14: error: method getThisOrNull() is already defined in class test
private Object getThisOrNull() {
^
1 error
Clearly in an ideal world I wouldn't write it like that to begin with, but the problem is this code will be generated automatically by a tool that is not really semantically or syntactically enough to distinguish between the static vs non-static case.
So, how can I write some source code that, although byte for byte identical compiles and behaves differently in depending on the presence of the static modifier for the method?
This can be achieved with a trick and a bit of help from Java's reflection facilities. It's ugly, but it works:
import java.lang.reflect.Field;
public class test {
public void method1() {
System.out.println(getThisOrNull(new Object(){}));
}
public static void method2() {
System.out.println(getThisOrNull(new Object(){}));
}
private static Object getThisOrNull(final Object o) {
for (Field f: o.getClass().getDeclaredFields()) {
if (f.getType().equals(test.class)) {
try {
return f.get(o);
}
catch (IllegalAccessException e) {
// Omm nom nom...
}
}
}
return null;
}
public static void main(String[] args) {
test t = new test();
System.out.println(t);
t.method1();
t.method2();
}
}
This compiles and runs as hoped for:
test#183f74d
test#183f74d
null
The trick that makes this possible is the use of new Object(){}, which creates a new, anonymous class within the existing method that we're trying to figure out if it's static or not. The behaviour of this is subtly different between the two cases.
If the goal were just to figure out if the method is static or not we could write:
java.lang.reflect.Modifiers.isStatic(new Object(){}.getClass().getEnclosingMethod().getModifiers())
Since we want to get this (when available) we need to do something slightly different. Fortunately for us classes defined within the context of an instance of an object in Java get an implicit reference to the class that contains them. (Normally you'd access it with test.this syntax). We needed a way to access test.this if it existed, except we can't actually write test.this anywhere because it too would be syntactically invalid in the static case. It does however exist within the object, as a private member variable. This means that we can find it with reflection, which is what the getThisOrNull static method does with the local anonymous type.
The downside is that we create an anonymous class in every method we use this trick and it probably adds overheads, but if you're backed into a corner and looking for a way of doing this it does at least work.
I am developing an Eclipse RCP application and I recently started to use Groovy in it. So 99% of my code is still Java.
I read that it is possible to use Groovy to override and add methods to Java classes and I was able to test this by adding a method to the java.lang.String.
But this only works when I use the string in a Groovy class. The overridden method is not considered as being overridden in a Java class.
Here's some code:
/*
* This is a Java class
*/
public class CTabItem {
...
private API
...
public void setControl(Control control){
private API
}
}
/*
* This is also a Java class
*/
public class Control {
...
private API
...
}
/*
* This is also a Java class
*/
public class OtherClass {
...
private API
...
private void someMethodIDontKnow(){
Control control = new Control();
CTabItem tab = new CTabItem();
tab.setControl(control);
}
}
/*
* This is a Groovy class
*/
public class MyViewPart extends org.eclipse.ui.part.ViewPart {
....
public void createPartControl(Composite parent) {
/* parent (or the parent of parent) is a Control
which is set somewhere in a CTabItem to which
I don't get access */
}
}
I need to get the tab from the control. But since it's not me who instantiates MyViewPart, but some private API, I have no access to it.
Is there something Groovy could do for me here? Any suggestion or code is welcome. Thank you!
The short answer is: no, it's not possible if the code creating the object and calling the method is pure Java (i.e., non-Groovy) code. Groovy does its magic by intercepting all method calls on objects (both Java objects and Groovy objects) and using its ExpandoMetaClass to add the behavior. However, it can't change how pure Java code determines which method to call on a pure Java class. To see, run the following sample code:
// UseTheString.java (a pure Java class)
public class UseTheString {
public static void main(String[] arg) {
String s = "Hello world";
System.out.println(s);
System.out.println(s.substring(1));
ModifyStringClass.messWithMetaClasses(s);
System.out.println(s.substring(1));
}
}
and
// ModifyStringClass.groovy (a Groovy class)
class ModifyStringClass {
public static messWithMetaClasses(String t) {
java.lang.String.metaClass.substring = { int n -> "!" }
println(t.substring(1))
}
}
You'll get the output:
Hello world
ello world
!
ello world
As you can see, Groovy can override the method on a pure Java object if it is called from other Groovy code, but it can't change how the Java code uses it.
I have utility class U that depends on a library X, and must go in a package that will be used from programs with X available (where it should do its normal stuff), and places without X (where it should do nothing). Without splitting the class in two, I have found a simple pattern that solves this:
package foo;
import bar.MisteriousX;
public class U {
static private boolean isXPresent = false;
static {
try {
isXPresent = (null != U.class.getClassLoader().loadClass("bar.MisteriousX"));
} catch (Exception e) {
// loading of a sample X class failed: no X for you
}
}
public static void doSomething() {
if (isXPresent) {
new Runnable() {
public void run() {
System.err.println("X says " + MisteriousX.say());
}
}.run();
} else {
System.err.println("X is not there");
}
}
public static void main(String args[]) { doSomething(); }
}
With this pattern, U requires X present to compile, but works as expected when run with or without X present. Unless all accesses to the X library are inside internal classes, this code launches a classloader exception.
Questions: is import resolution guaranteed to work like this everywhere, or will it depend on JVM/ClassLoader implementation? Is there an established pattern for this? Is the above code-snippet too hackish to make it into production?
Generally, when a class is first loaded, then if it refers to a class which does not exist, that might lead to an error. So yes, having one class do the check and another class actually access the external package without reflection will work as intended, at least on all implementations I've seen so far. It doesn't have to be an inner class.
The Linking section in the JVM specs give great freedom to implementations. If you don't use the two-class approach, then the verification of U in an implementation using eager linking will cause an attempt to load X which results in a LinkageError. The specs don't require the references class to be verified as well, but neither does it forbid such early verification. It does however require that
any error detected during resolution must be thrown at a point in the
program that (directly or indirectly) uses a symbolic reference to the
class or interface.
Seems you should be safe to assume that the error is only thrown when you actually access your inner class. If you look at the history of this answer, you will find that I already changed my opinion twice, so there will be no guarantees that I read it correctly this time… :-/
I do a similar thing in jOOQ, in order to load optional logging framework dependencies. I share your feelings about the fact that this is a bit of a hack, though. A sample code snippet of field initialisation, depending on class availability:
public final class JooqLogger {
private org.slf4j.Logger slf4j;
private org.apache.log4j.Logger log4j;
private java.util.logging.Logger util;
public static JooqLogger getLogger(Class<?> clazz) {
JooqLogger result = new JooqLogger();
// Prioritise slf4j
try {
result.slf4j = org.slf4j.LoggerFactory.getLogger(clazz);
}
// If that's not on the classpath, try log4j instead
catch (Throwable e1) {
try {
result.log4j = org.apache.log4j.Logger.getLogger(clazz);
}
// If that's not on the classpath either, ignore most of logging
catch (Throwable e2) {
result.util= java.util.logging.Logger.getLogger(clazz.getName());
}
}
return result;
}
[...]
And then, later on, the logger switch, depending on previously loaded classes:
public boolean isTraceEnabled() {
if (slf4j != null) {
return slf4j.isTraceEnabled();
}
else if (log4j != null) {
return log4j.isTraceEnabled();
}
else {
return util.isLoggable(Level.FINER);
}
}
The rest of the source code can be seen here. In essence, I have a compile-time dependency to both slf4j, and log4j, which I render optional at runtime using a similar pattern as you.
This could cause problems in OSGi environments, for instance, where classloading is a bit more complex than when you just use the standard JDK / JRE classloading mechanisms. However, so far I wasn't made aware of any issues