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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.
It is known that using sun.misc.Unsafe#allocateInstance one can create an object without calling any class constructors.
Is it possible to do the opposite: given an existing instance, invoke a constructor on it?
Clarification: this is not the question about something I'd do in production code. I'm curious about JVM internals and crazy things that can still be done. Answers specific to some JVM version are welcome.
JVMS §2.9 forbids invocation of constructor on already initialized objects:
Instance initialization methods may be invoked only within the Java
Virtual Machine by the invokespecial instruction, and
they may be invoked only on uninitialized class instances.
However, it is still technically possible to invoke constructor on initialized object with JNI. CallVoidMethod function does not make difference between <init> and ordinary Java methods. Moreover, JNI specification hints that CallVoidMethod may be used to call a constructor, though it does not say whether an instance has to be initialized or not:
When these functions are used to call private methods and constructors, the method ID must be derived from the real class of obj, not from one of its superclasses.
I've verified that the following code works both in JDK 8 and JDK 9. JNI allows you to do unsafe things, but you should not rely on this in production applications.
ConstructorInvoker.java
public class ConstructorInvoker {
static {
System.loadLibrary("constructorInvoker");
}
public static native void invoke(Object instance);
}
constructorInvoker.c
#include <jni.h>
JNIEXPORT void JNICALL
Java_ConstructorInvoker_invoke(JNIEnv* env, jclass self, jobject instance) {
jclass cls = (*env)->GetObjectClass(env, instance);
jmethodID constructor = (*env)->GetMethodID(env, cls, "<init>", "()V");
(*env)->CallVoidMethod(env, instance, constructor);
}
TestObject.java
public class TestObject {
int x;
public TestObject() {
System.out.println("Constructor called");
x++;
}
public static void main(String[] args) {
TestObject obj = new TestObject();
System.out.println("x = " + obj.x); // x = 1
ConstructorInvoker.invoke(obj);
System.out.println("x = " + obj.x); // x = 2
}
}
It seems that with some (very dubious) tricks this is possible, even without going through a custom native library, by (ab)using method handles.
This method essentially tricks the JVM into thinking it is currently invoking a regular method instead of a constructor.
I just have to add a mandatory "this is probably not a good idea", but this is the only way I found for doing this. I also can't attest to how this behaves on different JVMs.
Prerequisites
To do this, an instance of sun.misc.Unsafe is needed. I will not go into detail about how to obtain this here since you already seem to have one, but this guide explains the process.
Step 1: Obtaining a trusted MethodHandles.Lookup
Next, a java.lang.invoke.MethodHandles$Lookup is needed to get the actual method handle for the constructor.
This class has a permission system which works through the allowedModes property in Lookup, which is set to a bunch of Flags. There is a special TRUSTED flag that circumvents all permission checks.
Unfortunately, the allowedModes field is filtered from reflection, so we cannot simply bypass the permissions by setting that value through reflection.
Even though reflecion filters can be circumvented aswell, there is a simpler way: Lookup contains a static field IMPL_LOOKUP, which holds a Lookup with those TRUSTED permissions. We can get this instance by using reflection and Unsafe:
var field = MethodHandles.Lookup.class.getDeclaredField("IMPL_LOOKUP");
var fieldOffset = unsafe.staticFieldOffset(field);
var lookup = (MethodHandles.Lookup) unsafe.getObject(MethodHandles.Lookup.class, fieldOffset);
We use Unsafe here instead of setAccessible and get, because going through reflection will cause issues with the module system in the newer java versions.
Step 2: Finding the constructor
Now we can get a MethodHandle for the constructor we want to invoke. We do this by using the Lookup we just obtained, just like a Lookup would be used normally.
var type = MethodType.methodType(Void.TYPE, <your constructor argument types>);
var constructor = lookup.findConstructor(<your class>, type);
Step 3: Getting the MemberName
While the signature of findConstructor only specifies that it returns a MethodHandle, it actuall returns a java.lang.invoke.DirectMethodHandle$Constructor. This type declares a initMethod field, which contains the java.lang.invoke.MemberName referencing our constructor. The MemberName type is not accessible from the outside, so all interaction with it happens through Unsafe.
We can obtain this MemberName in the same way we also obtained the Lookup:
var constructorClass = Class.forName("java.lang.invoke.DirectMethodHandle$Constructor");
val initMethodField = constructorClass.getDeclaredField("initMethod");
val initMethodFieldOffset = unsafe.objectFieldOffset(initMethodField);
var initMemberName = unsafe.getObject(constructor, initMethodFieldOffset)
Step 4: Tricking Java
The next step is the important part. While there are no physical barriers from the JVM that prevent you from invoking a constructor like any other method, MethodHandle has some checks in place to ensure that you are not doing something fishy.
Most of the checks are circumvented by using the TRUSTED Lookup, and there remains one final check:
The MemberName instance contains a bunch of flags that, among other things, tell the system what kind of member the MemberName is referring to. These flags are checked.
To circumvent this, we can simply change the flags using Unsafe:
var memberNameClass = Class.forName("java.lang.invoke.MemberName");
var flagsField = memberNameClass.getDeclaredField("flags");
var flagsFieldOffset = unsafe.objectFieldOffset(flagsField);
var flags = unsafe.getInt(initMemberName, flagsFieldOffset);
flags &= ~0x00020000; // remove "is constructor"
flags |= 0x00010000; // add "is (non-constructor) method"
unsafe.putInt(initMemberName, flagsFieldOffset, flags);
The values for the flags come from java.lang.invoke.MethodHandleNatives.Constants#MN_IS_METHOD and java.lang.invoke.MethodHandleNatives.Constants#MN_IS_CONSTRUCTOR.
Step 5: Obtaining a REF_invokeVirtual method handle
Now that we have a totally legit method that is not at all a constructor, we just need to obtain a regular method handle for invoking it. Luckly, MethodHandles.Lookup.class has a private method for turning a MemberName into a (Direct)MethodHandle for all kinds of invocations: getDirectMethod.
Ironically, we actually call this method using our all-powerful lookup.
First, we obtain the MethodHandle for getDirectMethod:
var getDirectMethodMethodHandle = lookup.findVirtual(
MethodHandles.Lookup.class,
"getDirectMethod",
MethodType.methodType(
MethodHandle.class,
byte.class,
Class.class,
memberNameClass,
MethodHandles.Lookup.class
)
);
we can now use this with our lookup, to obtain a MethodHandle for our MemberName:
var handle = (MethodHandle) getDirectMethod.invoke(lookup, (byte) 5, Test.class, member, lookup);
The (byte) 5 argument stands for "invoke virtual", and comes from java.lang.invoke.MethodHandleNatives.Constants#REF_invokeVirtual.
Step 6: Profit?
We can now use this handle like a regular MethodHandle, to invoke the constructor on any existing instance of that class:
handle.invoke(<instance>, <constructor arguments...>);
With this handle, the constructor can also be called multiple times, and the instance doesn't actually have to come from Unsafe#allocateInstance - an instance that was created just by using new works aswell.
A constructor is not an instance method, so no you can't invoke a constructor on an instance.
If you look at the reflection library, you'll see that the return type of Class.getConstructor() is Constructor, which doesn't have any methods that can accept a instance - its only relevant method is newInstance(), which doesn't accept a target instance; it creates one.
On the other hand, the return type of Class.getMethod() is Method, whose first parameter is the instance.
A Constructor is not a Method.
In the JVM spec for invokespecial:
An invokespecial instruction is type safe iff all of the following are true:
... (Stuff about non-init methods)
MethodName is <init>.
Descriptor specifies a void return type.
One can validly pop types matching the argument types given in Descriptor and an uninitialized type, UninitializedArg, off the incoming operand stack, yielding OperandStack.
...
If you've already initialized the instance, it's not an uninitialized type, so this will fail.
Note that other invoke* instructions (invokevirtual, invokeinterface, invokestatic, invokedynamic) explicitly preclude invocation of <init> methods, so invokespecial is the only way to invoke them.
From JLS Sec 8.8
Constructors are invoked by class instance creation expressions (§15.9), by the conversions and concatenations caused by the string concatenation operator +(§15.18.1), and by explicit constructor invocations from other constructors (§8.8.7).
...
Constructors are never invoked by method invocation expressions (§15.12).
So no, it's not possible.
If there is some common action you want to take in the constructor and elsewhere, put it into a method, and invoke that from the constructor.
For example:
public class Demo{
public String getReflectString() {
return "string from reflect";
}
public String reflectMethod() throws Exception {
Method method = ReflectCase.class.getMethod("getReflectString");
return (String) method.invoke(this);
}
}
Method Demo#getReflectString called by reflection，and I wanna find a way to get all the methods in the program that are called this way.
I came up with a solution that has not yet been implemented.
Read the contents of the class file, start from the instruction, find the Method#invoke
instruction, and then look back to find the instance that calls the method, and then get the instance corresponding Class and MethodName.
However, this method has a problem, if Class or MethodName or even Method are passed as a parameter, it is difficult to find the corresponding content.
On this basis, think of ways to improve, traverse all the methods, check the instructions of the method. If got Method#invoke instruction in method A(), and Class and MethodName are passed in as parameters, record method A() and the order of parameters. In the following check, if there is a method call A(), you can get the value passed to method A() Parameters, to obtain the required value.
However, this solution requires traversing the entire program's instructions, which can be very time-consuming when the program is very large. If method call too many levels, the implementation of the algorithm can be quite complicated. So would like to ask, the feasibility of this idea, or is there any better way to solve the problem.
Thanks in advance.
I have the following code:
public class BiPredicateTest {
public static void main(String[] args) {
BiPredicate<List<Integer>, Integer> listContains = List::contains;
List aList = Arrays.asList(10, 20, 30);
System.out.println(listContains.test(aList, 20)); // prints true magically?
}
}
In the statement listContains.test(aList, 20), how is it that the method "contains" is getting called on the first argument and the second argument is passed in as a parameter? Something equivalent to:
System.out.println(aList.contains(20));
In other words, how does the statement listContains.test(aList, 20) get translated to aList.contains(20)?
Is this how java 8 BiPredicate work? Could someone explain how the magic is happening (with some references)?
This is not a duplicate post. This differs from "What does “an Arbitrary Object of a Particular Type” mean in java 8?" in that its not explicitly passing method reference around. It is very clear how method reference is being passed around in the post you reference. The array instance on which the method is being called is passed as an argument to Arrays.sort(). In my case, how the method "contains" is being called on aList is not apparent. I am looking for a reference or explanation as to how its working.
It seems some individuals prefer to down vote instead of provide reference or explanation. They give the impression that they have knowledge but refuse to share it.
BiPredicate is an interface which has only one method, test.
public interface BiPredicate<A,B> {
boolean test(A a, B b);
}
Interfaces which have only one method are called functional interfaces. Previous to Java 8, you would often times have to implement these interfaces using an anonymous class, just to create a wrapper for a certain method call with the same signature. Like this:
BiPredicate<List<Integer>,Integer> listContains = new BiPredicate<>() {
#Override
public boolean test(List<Integer> list, Integer num) {
return list.contains(num);
}
};
In Java 8, method references were added, which allowed for a much shorter syntax and more efficient bytecode for this pattern. In a method reference, you can specify a method or constructor which has the same signature as the type arguments for the interface. When you make a method reference using a class type, it assigns the class type as the first generic argument of the functional interface being used. This means whatever parameter which uses that generic type will need to be an instance of the class.
Even if the instance method normally doesn't take any parameters, a method reference can still be used which takes an instance as the parameter. For example:
Predicate<String> pred = String::isEmpty;
pred.test(""); // true
For more information, see the Java Tutorial for Method References.
I was microbenchmarking some code (please be nice) and came across this puzzle: when reading a field using reflection, invoking the getter Method is faster than reading the Field.
Simple test class:
private static final class Foo {
public Foo(double val) {
this.val = val;
}
public double getVal() { return val; }
public final double val; // only public for demo purposes
}
We have two reflections:
Method m = Foo.class.getDeclaredMethod("getVal", null);
Field f = Foo.class.getDeclaredField("val");
Now I call the two reflections in a loop, invoke on the Method, and get on the Field. A first run is done to warm up the VM, a second run is done with 10M iterations. The Method invocation is consistently 30% faster, but why? Note that getDeclaredMethod and getDeclaredField are not called in the loop. They are called once and executed on the same object in the loop.
I also tried some minor variations: made the field non-final, transitive, non-public, etc. All of these combinations resulted in statistically similar performance.
Edit: This is on WinXP, Intel Core2 Duo, Sun JavaSE build 1.6.0_16-b01, running under jUnit4 and Eclipse.
My educated guess would be a difference in how getDeclaredField and getDeclaredMethod are implemented: While each time it is called, getDeclaredField would have to check for the variable type and size, in order to return an actual object or primitive type, getDeclaredMethod would return the pointer to one and the same method, which takes care of all the rest statically.
Edit:
My explanation is similar: A method is contained in memory only once for each class, while each object instance can have different property values. When you get the property value by executing a method call (still using only the method pointer), the compiler has optimized the method to access the parameter, knowing the exact class hierarchy etc., while when you get the property's value by using "get", you let reflections do the getter method's job, and there can obviously be no compiler optimization.
In your microbenchmark, the method invocation is faster because of optimizations made by the JVM/Hotspot with in your loop.
Change your microbenchmak:
Make a loop in which: read the value by Reflection, then increase 1 (for instance), and then assign to the same Field via Reflection. And outside the loop, make a final read and System.out.println it...
Execute the two variants (Field vs Method) and you will see that the real difference is just the opposite: Method invocations are actually 30-40% slower.
Regards
does this also imply thatdouble d = Foo.getVal() is 30% faster than double d = Foo.val?