Related
I'm curious why it's possible to call kotlin's constructor with default param but not a method from java code?
For example:
class Test(val test: String = "")
Java client:
void test() {
new Test();
}
It's ok.
But if i want to do the same trick with method, it's not possible:
class Test {
fun x(x: Int = 5) { }
}
Java client, compilation error:
void test() {
new Test().x();
}
In the decompiled to java bytecode of the method i see
x$default
method. It is static and I can't call it from java(idea doesn't allow me to do it). And only addition of
#JvmOverloads
annotation on the kotlin's method with default arg creates one more method which is accessible from the java side.
The question is why there are two approaches how to call kotlin's defaults from java? Why not to do everything accessible/not accessible by default? Is it a bad design or there was serious reason to do so?
I believe, the considerations behind the current design were based on the fact that presence of a default (no-argument) constructor in a class is a widely used convention on JVM: there are many libraries and frameworks that rely on it (e.g. dependency injection tools, JSR-305). So, generating a no-argument constructor when there are default values for all of the parameters supports these use cases and might be expected by the users.
On contrary, functions usually don't have default values for all of their parameters, and I don't think it's needed by any idiom in the Java world. Moreover, you can expect many functions with default values in Kotlin classes, and generating the overloads for them by default would result into undesirable method count growth, which is in particular important for Android.
I have come across the #JvmSynthetic annotation in kotlin-stdlib, and I'm wondering what it is for, but, unfortunately, it is undocumented. (UPD: it was at that moment)
As far as I understand, applying it to a program element will add the synthetic modifier to the corresponding bytecode elements. As a consequence, the element becomes invisible from Java:
class MyClass {
#JvmSynthetic
fun f() { }
}
Somewhere in Java code:
MyClass c = new MyClass();
c.f() // Error: cannot resolve method f()
But the same elements are still visible in Kotlin code:
val c = MyClass()
c.f() // OK
Is hiding declarations from non-Kotlin sources a valid use of #JvmSynthetic? Is it the intended use? What are the other appropriate use cases?
Since #JvmSynthetic hides functions from Java, they cannot be overridden in Java either (and when it comes to an abstract member, the calls then result into AbstractMethodError). Given that, can I use #JvmSynthetic to prohibit overriding members of a Kotlin class in Java sources?
In plain Java, synthetic methods are generated by the javac compiler. Normally the compiler must create synthetic methods on nested classes, when fields specified with the private modifier are accessed by the enclosing class.
Given the following class in java:
public final class SyntheticSample
{
public static void main(final String[] args)
{
SyntheticSample.Nested nested = new SyntheticSample.Nested();
out.println("String: " + nested.syntheticString);
}
private static final class Nested
{
private String syntheticString = "I'll become a method!";
}
}
when the SyntheticSample class accesses the nested.syntheticString field, it is indeed calling a static synthetic method generated by the compiler (named something like access$100).
Even if Kotlin exposes a #JvmSynthetic annotation that is able to "force" the creation of synthetic methods, I advice to not using it in normal "user" code. Synthetic methods are low-level tricks made by the compiler, and we should never rely on such things in everyday code. I think it's there to support other parts of the standard library, but you should ask the JetBrains guys directly if you're curious (try on the official Kotlin Discussion Forum)
First, to answer what synthetic methods actually are, let's have a look at the Java language specification:
11. A construct emitted by a Java compiler must be marked as synthetic if it does not correspond to a construct declared explicitly or implicitly in source code, unless the emitted construct is a class initialization method (JVMS ยง2.9).
The #JvmSynthetic annotation does exactly that: prevent access from source code. The method will still appear in reflection and is then marked as synthetic.
More precisely, from the Kotlin documentation (emphasis mine):
#JvmSynthetic
Sets ACC_SYNTHETIC flag on the annotated target in the Java bytecode.
Synthetic targets become inaccessible for Java sources at compile time while still being accessible for Kotlin sources. Marking target as synthetic is a binary compatible change, already compiled Java code will be able to access such target.
This annotation is intended for rare cases when API designer needs to hide Kotlin-specific target from Java API while keeping it a part of Kotlin API so the resulting API is idiomatic for both.
As described in the last paragraph, #JvmSynthetic is a tool for API design, which lets a library writer avoid automatic generation of Java equivalents. Probably the most popular use cases are Kotlin-only features, such as operator overloading, componentN() methods or properties, which may have a more idiomatic way to be exposed in Java.
It is noteworthy that the target of this annotations are property setters/getters, functions and fields -- basically everything that translates in Java to a method.
#Target([
AnnotationTarget.FUNCTION,
AnnotationTarget.PROPERTY_GETTER,
AnnotationTarget.PROPERTY_SETTER,
AnnotationTarget.FIELD])
annotation actual class JvmSynthetic
Are there currently (Java 6) things you can do in Java bytecode that you can't do from within the Java language?
I know both are Turing complete, so read "can do" as "can do significantly faster/better, or just in a different way".
I'm thinking of extra bytecodes like invokedynamic, which can't be generated using Java, except that specific one is for a future version.
After working with Java byte code for quite a while and doing some additional research on this matter, here is a summary of my findings:
Execute code in a constructor before calling a super constructor or auxiliary constructor
In the Java programming language (JPL), a constructor's first statement must be an invocation of a super constructor or another constructor of the same class. This is not true for Java byte code (JBC). Within byte code, it is absolutely legitimate to execute any code before a constructor, as long as:
Another compatible constructor is called at some time after this code block.
This call is not within a conditional statement.
Before this constructor call, no field of the constructed instance is read and none of its methods is invoked. This implies the next item.
Set instance fields before calling a super constructor or auxiliary constructor
As mentioned before, it is perfectly legal to set a field value of an instance before calling another constructor. There even exists a legacy hack which makes it able to exploit this "feature" in Java versions before 6:
class Foo {
public String s;
public Foo() {
System.out.println(s);
}
}
class Bar extends Foo {
public Bar() {
this(s = "Hello World!");
}
private Bar(String helper) {
super();
}
}
This way, a field could be set before the super constructor is invoked which is however not longer possible. In JBC, this behavior can still be implemented.
Branch a super constructor call
In Java, it is not possible to define a constructor call like
class Foo {
Foo() { }
Foo(Void v) { }
}
class Bar() {
if(System.currentTimeMillis() % 2 == 0) {
super();
} else {
super(null);
}
}
Until Java 7u23, the HotSpot VM's verifier did however miss this check which is why it was possible. This was used by several code generation tools as a sort of a hack but it is not longer legal to implement a class like this.
The latter was merely a bug in this compiler version. In newer compiler versions, this is again possible.
Define a class without any constructor
The Java compiler will always implement at least one constructor for any class. In Java byte code, this is not required. This allows the creation of classes that cannot be constructed even when using reflection. However, using sun.misc.Unsafe still allows for the creation of such instances.
Define methods with identical signature but with different return type
In the JPL, a method is identified as unique by its name and its raw parameter types. In JBC, the raw return type is additionally considered.
Define fields that do not differ by name but only by type
A class file can contain several fields of the same name as long as they declare a different field type. The JVM always refers to a field as a tuple of name and type.
Throw undeclared checked exceptions without catching them
The Java runtime and the Java byte code are not aware of the concept of checked exceptions. It is only the Java compiler that verifies that checked exceptions are always either caught or declared if they are thrown.
Use dynamic method invocation outside of lambda expressions
The so-called dynamic method invocation can be used for anything, not only for Java's lambda expressions. Using this feature allows for example to switch out execution logic at runtime. Many dynamic programming languages that boil down to JBC improved their performance by using this instruction. In Java byte code, you could also emulate lambda expressions in Java 7 where the compiler did not yet allow for any use of dynamic method invocation while the JVM already understood the instruction.
Use identifiers that are not normally considered legal
Ever fancied using spaces and a line break in your method's name? Create your own JBC and good luck for code review. The only illegal characters for identifiers are ., ;, [ and /. Additionally, methods that are not named <init> or <clinit> cannot contain < and >.
Reassign final parameters or the this reference
final parameters do not exist in JBC and can consequently be reassigned. Any parameter, including the this reference is only stored in a simple array within the JVM what allows to reassign the this reference at index 0 within a single method frame.
Reassign final fields
As long as a final field is assigned within a constructor, it is legal to reassign this value or even not assign a value at all. Therefore, the following two constructors are legal:
class Foo {
final int bar;
Foo() { } // bar == 0
Foo(Void v) { // bar == 2
bar = 1;
bar = 2;
}
}
For static final fields, it is even allowed to reassign the fields outside of
the class initializer.
Treat constructors and the class initializer as if they were methods
This is more of a conceptional feature but constructors are not treated any differently within JBC than normal methods. It is only the JVM's verifier that assures that constructors call another legal constructor. Other than that, it is merely a Java naming convention that constructors must be called <init> and that the class initializer is called <clinit>. Besides this difference, the representation of methods and constructors is identical. As Holger pointed out in a comment, you can even define constructors with return types other than void or a class initializer with arguments, even though it is not possible to call these methods.
Create asymmetric records*.
When creating a record
record Foo(Object bar) { }
javac will generate a class file with a single field named bar, an accessor method named bar() and a constructor taking a single Object. Additionally, a record attribute for bar is added. By manually generating a record, it is possible to create, a different constructor shape, to skip the field and to implement the accessor differently. At the same time, it is still possible to make the reflection API believe that the class represents an actual record.
Call any super method (until Java 1.1)
However, this is only possible for Java versions 1 and 1.1. In JBC, methods are always dispatched on an explicit target type. This means that for
class Foo {
void baz() { System.out.println("Foo"); }
}
class Bar extends Foo {
#Override
void baz() { System.out.println("Bar"); }
}
class Qux extends Bar {
#Override
void baz() { System.out.println("Qux"); }
}
it was possible to implement Qux#baz to invoke Foo#baz while jumping over Bar#baz. While it is still possible to define an explicit invocation to call another super method implementation than that of the direct super class, this does no longer have any effect in Java versions after 1.1. In Java 1.1, this behavior was controlled by setting the ACC_SUPER flag which would enable the same behavior that only calls the direct super class's implementation.
Define a non-virtual call of a method that is declared in the same class
In Java, it is not possible to define a class
class Foo {
void foo() {
bar();
}
void bar() { }
}
class Bar extends Foo {
#Override void bar() {
throw new RuntimeException();
}
}
The above code will always result in a RuntimeException when foo is invoked on an instance of Bar. It is not possible to define the Foo::foo method to invoke its own bar method which is defined in Foo. As bar is a non-private instance method, the call is always virtual. With byte code, one can however define the invocation to use the INVOKESPECIAL opcode which directly links the bar method call in Foo::foo to Foo's version. This opcode is normally used to implement super method invocations but you can reuse the opcode to implement the described behavior.
Fine-grain type annotations
In Java, annotations are applied according to their #Target that the annotations declares. Using byte code manipulation, it is possible to define annotations independently of this control. Also, it is for example possible to annotate a parameter type without annotating the parameter even if the #Target annotation applies to both elements.
Define any attribute for a type or its members
Within the Java language, it is only possible to define annotations for fields, methods or classes. In JBC, you can basically embed any information into the Java classes. In order to make use of this information, you can however no longer rely on the Java class loading mechanism but you need to extract the meta information by yourself.
Overflow and implicitly assign byte, short, char and boolean values
The latter primitive types are not normally known in JBC but are only defined for array types or for field and method descriptors. Within byte code instructions, all of the named types take the space 32 bit which allows to represent them as int. Officially, only the int, float, long and double types exist within byte code which all need explicit conversion by the rule of the JVM's verifier.
Not release a monitor
A synchronized block is actually made up of two statements, one to acquire and one to release a monitor. In JBC, you can acquire one without releasing it.
Note: In recent implementations of HotSpot, this instead leads to an IllegalMonitorStateException at the end of a method or to an implicit release if the method is terminated by an exception itself.
Add more than one return statement to a type initializer
In Java, even a trivial type initializer such as
class Foo {
static {
return;
}
}
is illegal. In byte code, the type initializer is treated just as any other method, i.e. return statements can be defined anywhere.
Create irreducible loops
The Java compiler converts loops to goto statements in Java byte code. Such statements can be used to create irreducible loops, which the Java compiler never does.
Define a recursive catch block
In Java byte code, you can define a block:
try {
throw new Exception();
} catch (Exception e) {
<goto on exception>
throw Exception();
}
A similar statement is created implicitly when using a synchronized block in Java where any exception while releasing a monitor returns to the instruction for releasing this monitor. Normally, no exception should occur on such an instruction but if it would (e.g. the deprecated ThreadDeath), the monitor would still be released.
Call any default method
The Java compiler requires several conditions to be fulfilled in order to allow a default method's invocation:
The method must be the most specific one (must not be overridden by a sub interface that is implemented by any type, including super types).
The default method's interface type must be implemented directly by the class that is calling the default method. However, if interface B extends interface A but does not override a method in A, the method can still be invoked.
For Java byte code, only the second condition counts. The first one is however irrelevant.
Invoke a super method on an instance that is not this
The Java compiler only allows to invoke a super (or interface default) method on instances of this. In byte code, it is however also possible to invoke the super method on an instance of the same type similar to the following:
class Foo {
void m(Foo f) {
f.super.toString(); // calls Object::toString
}
public String toString() {
return "foo";
}
}
Access synthetic members
In Java byte code, it is possible to access synthetic members directly. For example, consider how in the following example the outer instance of another Bar instance is accessed:
class Foo {
class Bar {
void bar(Bar bar) {
Foo foo = bar.Foo.this;
}
}
}
This is generally true for any synthetic field, class or method.
Define out-of-sync generic type information
While the Java runtime does not process generic types (after the Java compiler applies type erasure), this information is still attcheched to a compiled class as meta information and made accessible via the reflection API.
The verifier does not check the consistency of these meta data String-encoded values. It is therefore possible to define information on generic types that does not match the erasure. As a concequence, the following assertings can be true:
Method method = ...
assertTrue(method.getParameterTypes() != method.getGenericParameterTypes());
Field field = ...
assertTrue(field.getFieldType() == String.class);
assertTrue(field.getGenericFieldType() == Integer.class);
Also, the signature can be defined as invalid such that a runtime exception is thrown. This exception is thrown when the information is accessed for the first time as it is evaluated lazily. (Similar to annotation values with an error.)
Append parameter meta information only for certain methods
The Java compiler allows for embedding parameter name and modifier information when compiling a class with the parameter flag enabled. In the Java class file format, this information is however stored per-method what makes it possible to only embed such method information for certain methods.
Mess things up and hard-crash your JVM
As an example, in Java byte code, you can define to invoke any method on any type. Usually, the verifier will complain if a type does not known of such a method. However, if you invoke an unknown method on an array, I found a bug in some JVM version where the verifier will miss this and your JVM will finish off once the instruction is invoked. This is hardly a feature though, but it is technically something that is not possible with javac compiled Java. Java has some sort of double validation. The first validation is applied by the Java compiler, the second one by the JVM when a class is loaded. By skipping the compiler, you might find a weak spot in the verifier's validation. This is rather a general statement than a feature, though.
Annotate a constructor's receiver type when there is no outer class
Since Java 8, non-static methods and constructors of inner classes can declare a receiver type and annotate these types. Constructors of top-level classes cannot annotate their receiver type as they most not declare one.
class Foo {
class Bar {
Bar(#TypeAnnotation Foo Foo.this) { }
}
Foo() { } // Must not declare a receiver type
}
Since Foo.class.getDeclaredConstructor().getAnnotatedReceiverType() does however return an AnnotatedType representing Foo, it is possible to include type annotations for Foo's constructor directly in the class file where these annotations are later read by the reflection API.
Use unused / legacy byte code instructions
Since others named it, I will include it as well. Java was formerly making use of subroutines by the JSR and RET statements. JBC even knew its own type of a return address for this purpose. However, the use of subroutines did overcomplicate static code analysis which is why these instructions are not longer used. Instead, the Java compiler will duplicate code it compiles. However, this basically creates identical logic which is why I do not really consider it to achieve something different. Similarly, you could for example add the NOOP byte code instruction which is not used by the Java compiler either but this would not really allow you to achieve something new either. As pointed out in the context, these mentioned "feature instructions" are now removed from the set of legal opcodes which does render them even less of a feature.
As far as I know there are no major features in the bytecodes supported by Java 6 that are not also accessible from Java source code. The main reason for this is obviously that the Java bytecode was designed with the Java language in mind.
There are some features that are not produced by modern Java compilers, however:
The ACC_SUPER flag:
This is a flag that can be set on a class and specifies how a specific corner case of the invokespecial bytecode is handled for this class. It is set by all modern Java compilers (where "modern" is >= Java 1.1, if I remember correctly) and only ancient Java compilers produced class files where this was un-set. This flag exists only for backwards-compatibility reasons. Note that starting with Java 7u51, ACC_SUPER is ignored completely due to security reasons.
The jsr/ret bytecodes.
These bytecodes were used to implement sub-routines (mostly for implementing finally blocks). They are no longer produced since Java 6. The reason for their deprecation is that they complicate static verification a lot for no great gain (i.e. code that uses can almost always be re-implemented with normal jumps with very little overhead).
Having two methods in a class that only differ in return type.
The Java language specification does not allow two methods in the same class when they differ only in their return type (i.e. same name, same argument list, ...). The JVM specification however, has no such restriction, so a class file can contain two such methods, there's just no way to produce such a class file using the normal Java compiler. There's a nice example/explanation in this answer.
Here are some features that can be done in Java bytecode but not in Java source code:
Throwing a checked exception from a method without declaring that the method throws it. The checked and unchecked exceptions are a thing which is checked only by the Java compiler, not the JVM. Because of this for example Scala can throw checked exceptions from methods without declaring them. Though with Java generics there is a workaround called sneaky throw.
Having two methods in a class that only differ in return type, as already mentioned in Joachim's answer: The Java language specification does not allow two methods in the same class when they differ only in their return type (i.e. same name, same argument list, ...). The JVM specification however, has no such restriction, so a class file can contain two such methods, there's just no way to produce such a class file using the normal Java compiler. There's a nice example/explanation in this answer.
GOTO can be used with labels to create your own control structures (other than for while etc)
You can override the this local variable inside a method
Combining both of these you can create create tail call optimised bytecode (I do this in JCompilo)
As a related point you can get parameter name for methods if compiled with debug (Paranamer does this by reading the bytecode
Maybe section 7A in this document is of interest, although it's about bytecode pitfalls rather than bytecode features.
In Java language the first statement in a constructor must be a call to the super class constructor. Bytecode does not have this limitation, instead the rule is that the super class constructor or another constructor in the same class must be called for the object before accessing the members. This should allow more freedom such as:
Create an instance of another object, store it in a local variable (or stack) and pass it as a parameter to super class constructor while still keeping the reference in that variable for other use.
Call different other constructors based on a condition. This should be possible: How to call a different constructor conditionally in Java?
I have not tested these, so please correct me if I'm wrong.
Something you can do with byte code, rather than plain Java code, is generate code which can loaded and run without a compiler. Many systems have JRE rather than JDK and if you want to generate code dynamically it may be better, if not easier, to generate byte code instead of Java code has to be compiled before it can be used.
I wrote a bytecode optimizer when I was a I-Play, (it was designed to reduce the code size for J2ME applications). One feature I added was the ability to use inline bytecode (similar to inline assembly language in C++). I managed to reduce the size of a function that was part of a library method by using the DUP instruction, since I need the value twice. I also had zero byte instructions (if you are calling a method that takes a char and you want to pass an int, that you know does not need to be cast I added int2char(var) to replace char(var) and it would remove the i2c instruction to reduce the size of the code. I also made it do float a = 2.3; float b = 3.4; float c = a + b; and that would be converted to fixed point (faster, and also some J2ME did not support floating point).
In Java, if you attempt to override a public method with a protected method (or any other reduction in access), you get an error: "attempting to assign weaker access privileges". If you do it with JVM bytecode, the verifier is fine with it, and you can call these methods via the parent class as if they were public.
Is there something like Java's annotations in C++ ?
For example, the #Override annotation marks a function that it overrides another function, and if it wouldn't, it would give an error at compile time.
I am looking for something like this in C++.
C++11 provides support for generalized attributes, which can be seen as superset of Java annotations, as they can be applied not just to variables/functions, but also to statements, for example. But C++11 defines only syntax for generalized attributes, not means for user to define them.
This article gives good overview of generalized attributes: http://www.codesynthesis.com/~boris/blog/2012/04/18/cxx11-generalized-attributes/
GCC supports this feature from version 4.8, according to: http://gcc.gnu.org/projects/cxx0x.html
To implement support for user-defined attributes, compiler plugins are promising, especially based on high-level language integration, like https://fedorahosted.org/gcc-python-plugin/
C++0x will have this feature, where you can explicitly specify whether a member function is meant to override a base class' function, use a default implementation generated by the compiler and much more.
There is C++0x, which has the override 'annotation'. Or, if you wanted to achieve more of the Java "interface" like-code that errors if you don't implement methods, you could use an abstract class:
class Base {
public:
virtual void foo() = 0;
};
class Extended : public Base {
public:
void foo2() {
cout << "hi" << endl;
};
int main() {
Extended e;
e.foo();
}
This will result in a compiler error if you don't override foo in the base class. The issue, however, is that the base class can't have it's own implementation.
There's nothing in the language for this. The best you could hope for is a compiler-specific option. I'd start by checking the documentation for "pragma" for your compiler.
I'm not sure what JAVA provides in general, but for the specific functionality you mentioned, C++ has the override keyword:
class Derived : public Base {
void foo() override { ... }
};
You'll get a helpful compiler error message if Base doesn't have a corresponding virtual void foo().
Another functionally-similar keyword is final, which can be used to say that the function is an override that can't be further overridden in further-derived classes. (The same keyword can be used to say a class can't be derived from).
I'm writing a library that needs to have some code if a particular library is included. Since this code is scattered all around the project, it would be nice if users didn't have to comment/uncomment everything themselves.
In C, this would be easy enough with a #define in a header, and then code blocks surrounded with #ifdefs. Of course, Java doesn't have the C preprocessor...
To clarify - several external libraries will be distributed with mine. I do not want to have to include them all to minimize my executable size. If a developer does include a library, I need to be able to use it, and if not, then it can just be ignored.
What is the best way to do this in Java?
There's no way to do what you want from within Java. You could preprocess the Java source files, but that's outside the scope of Java.
Can you not abstract the differences and then vary the implementation?
Based on your clarification, it sounds like you might be able to create a factory method that will return either an object from one of the external libraries or a "stub" class whose functions will do what you would have done in the "not-available" conditional code.
As other have said, there is no such thing as #define/#ifdef in Java. But regarding your problem of having optional external libraries, which you would use, if present, and not use if not, using proxy classes might be an option (if the library interfaces aren't too big).
I had to do this once for the Mac OS X specific extensions for AWT/Swing (found in com.apple.eawt.*). The classes are, of course, only on the class-path if the application is running on Mac OS. To be able to use them but still allow the same app to be used on other platforms, I wrote simple proxy classes, which just offered the same methods as the original EAWT classes. Internally, the proxies used some reflection to determine if the real classes were on the class-path and would pass through all method calls. By using the java.lang.reflect.Proxy class, you can even create and pass around objects of a type defined in the external library, without having it available at compile time.
For example, the proxy for com.apple.eawt.ApplicationListener looked like this:
public class ApplicationListener {
private static Class<?> nativeClass;
static Class<?> getNativeClass() {
try {
if (ApplicationListener.nativeClass == null) {
ApplicationListener.nativeClass = Class.forName("com.apple.eawt.ApplicationListener");
}
return ApplicationListener.nativeClass;
} catch (ClassNotFoundException ex) {
throw new RuntimeException("This system does not support the Apple EAWT!", ex);
}
}
private Object nativeObject;
public ApplicationListener() {
Class<?> nativeClass = ApplicationListener.getNativeClass();
this.nativeObject = Proxy.newProxyInstance(nativeClass.getClassLoader(), new Class<?>[] {
nativeClass
}, new InvocationHandler() {
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
String methodName = method.getName();
ApplicationEvent event = new ApplicationEvent(args[0]);
if (methodName.equals("handleReOpenApplication")) {
ApplicationListener.this.handleReOpenApplication(event);
} else if (methodName.equals("handleQuit")) {
ApplicationListener.this.handleQuit(event);
} else if (methodName.equals("handlePrintFile")) {
ApplicationListener.this.handlePrintFile(event);
} else if (methodName.equals("handlePreferences")) {
ApplicationListener.this.handlePreferences(event);
} else if (methodName.equals("handleOpenFile")) {
ApplicationListener.this.handleOpenFile(event);
} else if (methodName.equals("handleOpenApplication")) {
ApplicationListener.this.handleOpenApplication(event);
} else if (methodName.equals("handleAbout")) {
ApplicationListener.this.handleAbout(event);
}
return null;
}
});
}
Object getNativeObject() {
return this.nativeObject;
}
// followed by abstract definitions of all handle...(ApplicationEvent) methods
}
All this only makes sense, if you need just a few classes from an external library, because you have to do everything via reflection at runtime. For larger libraries, you probably would need some way to automate the generation of the proxies. But then, if you really are that dependent on a large external library, you should just require it at compile time.
Comment by Peter Lawrey: (Sorry to edit, its very hard to put code into a comment)
The follow example is generic by method so you don't need to know all the methods involved. You can also make this generic by class so you only need one InvocationHandler class coded to cover all cases.
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
String methodName = method.getName();
ApplicationEvent event = new ApplicationEvent(args[0]);
Method method = ApplicationListener.class.getMethod(methodName, ApplicationEvent.class);
return method.invoke(ApplicationListener.this, event);
}
In Java one could use a variety of approaches to achieve the same result:
Dependency Injection
Annotations
Reflection
The Java way is to put behaviour that varies into a set of separate classes abstracted through an interface, then plug the required class at run time. See also:
Factory pattern
Builder pattern
Strategy pattern
Well, Java syntax is close enough to C that you could simply use the C preprocessor, which is usually shipped as a separate executable.
But Java isn't really about doing things at compile time anyway. The way I've handled similar situations before is with reflection. In your case, since your calls to the possibly-non-present library are scattered throughout the code, I would make a wrapper class, replace all the calls to the library with calls to the wrapper class, and then use reflection inside the wrapper class to invoke on the library if it is present.
Use a constant:
This week we create some constants
that have all of the benefits of using
the C preprocessor's facilities to
define compile-time constants and
conditionally compiled code.
Java has gotten rid of the entire
notion of a textual preprocessor (if
you take Java as a "descendent" of
C/C++). We can, however, get the best
benefits of at least some of the C
preprocessor's features in Java:
constants and conditional compilation.
I don't believe that there really is such a thing. Most true Java users will tell you that this is a Good Thing, and that relying on conditional compilation should be avoided at almost all costs.
I'm don't really agree with them...
You CAN use constants that can be defined from the compile line, and that will have some of the effect, but not really all. (For example, you can't have things that don't compile, but you still want, inside #if 0... (and no, comments don't always solve that problem, because nesting comments can be tricky...)).
I think that most people will tell you to use some form of inheritance to do this, but that can be very ugly as well, with lots of repeated code...
That said, you CAN always just set up your IDE to throw your java through the pre-processor before sending it to javac...
"to minimize my executable size"
What do you mean by "executable size"?
If you mean the amount of code loaded at runtime, then you can conditionally load classes through the classloader. So you distribute your alternative code no matter what, but it's only actually loaded if the library that it stands in for is missing. You can use an Adapter (or similar) to encapsulate the API, to make sure that almost all of your code is exactly the same either way, and one of two wrapper classes is loaded according to your case. The Java security SPI might give you some ideas how this can be structured and implemented.
If you mean the size of your .jar file, then you can do the above, but tell your developers how to strip the unnecessary classes out of the jar, in the case where they know they aren't going to be needed.
I have one more best way to say.
What you need is a final variable.
public static final boolean LibraryIncluded= false; //or true - manually set this
Then inside the code say as
if(LibraryIncluded){
//do what you want to do if library is included
}
else
{
//do if you want anything to do if the library is not included
}
This will work as #ifdef. Any one of the blocks will be present in the executable code. Other will be eliminated in the compile time itself
Use properties to do this kind of thing.
Use things like Class.forName to identify the class.
Do not use if-statements when you can trivially translate a property directly to a class.
Depending on what you are doing (not quite enough information) you could do something like this:
interface Foo
{
void foo();
}
class FakeFoo
implements Foo
{
public void foo()
{
// do nothing
}
}
class RealFoo
{
public void foo()
{
// do something
}
}
and then provide a class to abstract the instantiation:
class FooFactory
{
public static Foo makeFoo()
{
final String name;
final FooClass fooClass;
final Foo foo;
name = System.getProperty("foo.class");
fooClass = Class.forName(name);
foo = (Foo)fooClass.newInstance();
return (foo);
}
}
Then run java with -Dfoo.name=RealFoo|FakeFoo
Ignored the exception handling in the makeFoo method and you can do it other ways... but the idea is the same.
That way you compile both versions of the Foo subclasses and let the developer choose at runtime which they wish to use.
I see you specifying two mutually exclusive problems here (or, more likely, you have chosen one and I'm just not understanding which choice you've made).
You have to make a choice: Are you shipping two versions of your source code (one if the library exists, and one if it does not), or are you shipping a single version and expecting it to work with the library if the library exists.
If you want a single version to detect the library's existence and use it if available, then you MUST have all the code to access it in your distributed code--you cannot trim it out. Since you are equating your problem with using a #define, I assumed this was not your goal--you want to ship 2 versions (The only way #define can work)
So, with 2 versions you can define a libraryInterface. This can either be an object that wraps your library and forwards all the calls to the library for you or an interface--in either case this object MUST exist at compile time for both modes.
public LibraryInterface getLibrary()
{
if(LIBRARY_EXISTS) // final boolean
{
// Instantiate your wrapper class or reflectively create an instance
return library;
}
return null;
}
Now, when you want to USE your library (cases where you would have had a #ifdef in C) you have this:
if(LIBRARY_EXISTS)
library.doFunc()
Library is an interface that exists in both cases. Since it's always protected by LIBRARY_EXISTS, it will compile out (should never even load into your class loader--but that's implementation dependent).
If your library is a pre-packaged library provided by a 3rd party, you may have to make Library a wrapper class that forwards it's calls to your library. Since your library wrapper is never instantiated if LIBRARY_EXISTS is false, it shouldn't even be loaded at runtime (Heck, it shouldn't even be compiled in if the JVM is smart enough since it's always protected by a final constant.) but remember that the wrapper MUST be available at compile time in both cases.
If it helps have a look at j2me polish or Using preprocessor directives in BlackBerry JDE plugin for eclipse?
this is for mobiles app but this can be reused no ?