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Monthly Archives: January 2016

Creating a proxy object using cglib

In the previous post I was talking about the standard Java based proxy objects. These can be used when you want to have a method invocation handler on an object that implements an interface. The Java reflection proxy creation demands that you have an object that implements the interface. The object we want to proxy is out of our hand, it does not implement the interface that we want to invoke from our handler and still we want to have a proxy.

When do we need proxy to objects w/o interface?

This is a very common case. If ever we have a JPA implementation e.g. Hibernate that implements lazy loading of the records. For example the audit log records are stored in a table and each record, except the first one has a reference to the previous item. Something like

class LinkedAuditLogRecord {
  LinkedAuditLogRecord previous;
  AuditLogRecord actualRecord;
}

Loading a record via JPA will return an object LinkedAuditLogRecord which contains the previous record as an object and so on until the first one that probably has null in the field named previos. (This is not an actual code.) Any JPA implementation grabbing and loading the whole table from the start to the record of our interest would be an extremely poor implementation. Instead the persistence layer loads the actual record only and creates a proxy object extending LinkedAuditLogRecord and that is what the field previous is going to be. The actual fields are usually private fields and if ever our code tries to access the previous record the proxy object will load it that time. This is lazy loading in short.

But how do the JPA implementations create proxies to objects of classes that do not implement interfaces? Java reflection proxy implementation can not do that and thus JPA implementation uses something different. What they usually use is cglib.

What is cglib

Cglib is an open source library that capable creating and loading class files in memory during Java run time. To do that it uses Java byte-code generation library ‘asm’, which is a very low level byte code creation tool. I will not dig that deep in this article.

How to use cglib

To create a proxy object using cglib is almost as simple as using the JDK reflection proxy API. I created the same code as the last week article, this time using cglib:

package proxy;

import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;

import java.lang.reflect.Method;

public class CglibProxyDemo {

    static class Original {
        public void originalMethod(String s) {
            System.out.println(s);
        }
    }

    static class Handler implements MethodInterceptor {
        private final Original original;

        public Handler(Original original) {
            this.original = original;
        }

        public Object intercept(Object o, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
            System.out.println("BEFORE");
            method.invoke(original, args);
            System.out.println("AFTER");
            return null;
        }
    }

    public static void main(String[] args){
        Original original = new Original();
        MethodInterceptor handler = new Handler(original);
        Original f = (Original) Enhancer.create(Original.class,handler);
        f.originalMethod("Hallo");
    }
}

The difference is that name of the classes are a bit different and we do not have an interface.

It is also important that the proxy class extends the original class and thus when the proxy object is created it invokes the constructor of the original class. In case this is resource hungry we may have some issue with that. However this is something that we can not circumvent. If we want to have a proxy object to an already existing class then we should have either an interface or we have to extend the original class, otherwise we just could not use the proxy object in place of the original one.

Java Dynamic Proxy

Proxy is a design pattern. We create and use proxy objects when we want to add or modify some functionality of an already existing class. The proxy object is used instead of the original one. Usually the proxy objects have the same methods as the original one and in Java proxy classes usually extend the original class. The proxy has a handle to the original object and can call the method on that.

This way proxy classes can implement many things in a convenient way:

  • logging when a method starts and stops
  • perform extra checks on arguments
  • mocking the behavior of the original class
  • implement lazy access to costly resources

without modifying the original code of the class. (The above list is not extensive, only examples.)

In practical applications the proxy class does not directly implement the functionality. Following the single responsibility principle the proxy class does only proxying and the actual behavior modification is implemented in handlers. When the proxy object is invoked instead of the original object the proxy decides if it has to invoke the original method or some handler. The handler may do its task and may also call the original method.

Even though the proxy pattern does not only apply into situation when the proxy object and proxy class is created during run-time, this is an especially interesting topic in Java. In this article I will focus on these proxies.

This is an advanced topic because it requires the use of the reflection class, or byte code manipulation or compiling Java code generated dynamically. Or all of these. To have a new class not available as a byte code yet during run-time will need the generation of the byte code, and a class loader that loads the byte code. To create the byte code you can use cglib or bytebuddy or the built in Java compiler.

When we think about the proxy classes and the handlers they invoke we can understand why the separation of responsibilities in this case is important. The proxy class is generated during run-time, but the handler invoked by the proxy class can be coded in the normal source code and compiled along the code of the whole program (compile time).

The easiest way to do this is to use the java.lang.reflect.Proxy class, which is part of the JDK. That class can create a proxy class or directly an instance of it. The use of the Java built-in proxy is easy. All you need to do is implement a java.lang.InvocationHandler so that the proxy object can invoke that. InvocationHandler interface is extremely simple. It contains only one method: invoke(). When invoke() is invoked the arguments contain the original object, which is proxied, the method that was invoked (as a reflection Method object) and the object array of the original arguments. A sample code demonstrate the use:

package proxy;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class JdkProxyDemo {

    interface If {
        void originalMethod(String s);
    }

    static class Original implements If {
        public void originalMethod(String s) {
            System.out.println(s);
        }
    }

    static class Handler implements InvocationHandler {
        private final If original;

        public Handler(If original) {
            this.original = original;
        }

        public Object invoke(Object proxy, Method method, Object[] args)
                throws IllegalAccessException, IllegalArgumentException,
                InvocationTargetException {
            System.out.println("BEFORE");
            method.invoke(original, args);
            System.out.println("AFTER");
            return null;
        }
    }

    public static void main(String[] args){
        Original original = new Original();
        Handler handler = new Handler(original);
        If f = (If) Proxy.newProxyInstance(If.class.getClassLoader(),
                new Class[] { If.class },
                handler);
        f.originalMethod("Hallo");
    }

}

If the handler wants to invoke the original method on the original object it has to have access it. This is not provided by the Java proxy implementation. You have to pass this argument to the handler instance yourself in your code. (Note that there is an object usually named proxy passed as an argument to the invocation handler. This is the proxy object that the Java reflection dynamically generate and not the object we want to proxy.) This way you are absolutely free to use a separate handler object for each original class or use some shared object that happens to know some way which original object to invoke if there is any method to invoke at all.

As a special case you can create an invocation handler and a proxy of an interface that does not have any original object. Even more it is not needed to have any class to implement the interface in the source code. The dynamically created proxy class will implement the interface.

What should you do if the class you want to proxy does not implement an interface? In that case you have to use some other proxy implementation. We will look at about that next week.

Value types in Java: why should they be immutable?

Value types need not be immutable. But they are.

In the previous post I discussed the difference between pointers and references in Java and how the method parameters are passed (passed-by-value or passed-by-reference). These are strongly related to value types that do not exist in Java (yet).

There is a proposal from John Rose, Brian Goetz, and Guy Steele detailing how value types will/may work in Java and also there are some good articles about it. I have read “Value Types: Revamping Java’s Type System” that I liked a lot and I recommend to read. If the proposal is too dense for you to follow the topic you can read that article first. It summarizes very much the background, what value types are, advantages, why it is a problem that Java does not implement value types and why it is not trivial. Even though the terminology “value type” may also be used to denote something different I will use it as it is used in the proposal and in the article.

How do we pass arguments vs. what do we store in variables

As you may recall from the previous article I detailed that Java passes method arguments by reference or by value depending on the type of the argument:

  • reference is passed when the argument is an object
  • by-value when the argument is primitive.

There are some comments on the original post and also on the JCG republish that complain about my terminology about passing an argument by-reference. The comments state that arguments are always passed by value because the variables already contain reference to the objects. In reality variables, however contain bits. Even though this is important to know how we imagine those bits, and what terminology we use when we communicate. We can either say that

  1. class variables contain objects and in that case we pass these objects to methods by-reference
  2. or we can say that the variables contain the reference and in that case we pass the value of the variables.

If we follow the thinking #1 then the argument passing is by-value and/or by-reference based on the actual nature of the argument (object or primitive). If we follow the thinking #2 then the variables store reference and/or values based on the nature of their type. I personally like to think that when I write

Triangle triangle;

then the variable triangle is a triangle and not a reference to a triangle. But it does not really matter what it is in my brain. In either of the cases #1 or #2 there is a different approach for class types and for primitives. If we introduce value types to the language the difference becomes more prevalent and important to understand.

Value types are immutable

I explained that the implicit argument passing based on type does not cause any issue because primitives are immutable and therefore, when passed as method argument, they could not be changed even if they were passed by reference. So we usually do not care. Value types are not different. Value types are also immutable because they are values and values do not change. For example the value of PI is 3.145926… and it never changes.

However, what does this immutability mean in programming? Values be real numbers, integers or compound value types are all represented in memory as bits. Bits in memory (unless memory is ROM) can be changed.

In case of an object immutability is fairly simple. There is an object somewhere in the universe that we can not alter. There can be numerous variables holding the object (having a reference to it) and the code can rely on the fact that the bits at the memory location where the actual value of the object is represented are not changed (more or less).

In case of value types this is a bit different and this difference comes from the different interpretation of the bits that represent a value type from the same bits when they may represent an object.

Value types have no identity

Value types do not have identity. You can not have two int variables holding the value 3 and distinguish one from the other. They hold the same value. This is the same when the type is more complex.

Say I have a value type that has two fields, like

ValueType TwoFields {
  int count;
  double size;
  }

and say I have two variables

 Twofields tF1 = new TwoFields(1,3.14)
 Twofields tF2 = new TwoFields(1,3.14)

I can not tell the variables tF1 and tF2 from other. If they were objects they would be equals to each other but not == to each other. In case of value types there is not == as they have no identity.

If TwoFields is immutable class I can not or should not write

 TwoFields tF;
  ...
 tF.count++;

or some similar construct. But I still can write

 TwoFields tF;
  ...
 tF = new TwoFields(tF.count+1, tF.size)

which leaves the original object intact. If TwoFields is a value type then either of the constructs, whichever is allowed, will create a new value.

Value types as arguments

How are value types passed as method argument then? Probably copying the value to the parameter variable. Possibly passing some reference. It is, however, up to the compiler (be it Java, or some other language). Why?

  • Value types are usually small. At least they should be small. A huge value type looses the advantages that value types deliver but have the disadvantages.
  • Value types are immutable so there is no problem copying them just like in case of primitives. They can be passed by value the same way as “everything in Java is passed by value”.
  • They have no identity, there can be no references to them.

But this is not only about passing them as arguments. This is also how variables are assigned. Look at the code

 Twofields tF1 = new TwoFields(1,3.14)
 Twofields tF2 = new TwoFields(1,3.14)

and compare it to

 Twofields tF1 = new TwoFields(1,3.14)
 Twofields tF2 = tF1

If TwoFields is a value type there should be no difference between the two versions. They have to produce the same result (though may not through the same code when compiled). In this respect there is no real difference between argument passing and variable assignment. Values are copied even if the actual variables as bits contain some references to some memory locations where the values are stored.

Summary

As I started the article: value types need not be immutable. This is not something that the language designers decide. They are free to implement something that is mutable, but in that case it will not be value type. Value types are immutable.

Pointers in Java

Are there pointers in Java? The short answer is “no, there are none” and this seems to be obvious for many developers. But why is it not that obvious for others?

That is because the references that Java uses to access objects are very similar to pointers. If you have experience with C programming before Java it may be easier to think about the values that are stored in the variables as pointers that point to some memory locations holding the objects. And it is more or less ok. More less than more but that is what we will look at now.

Difference between reference and pointer

As Brian Agnew summarized on stackoverflow there are two major differences.

  1. There is no pointer arithmetic
  2. References do not “point” to a memory location

Missing pointer arithmetic

When you have an array of a struct in C the memory allocated for the array contains the content of the structures one after the other. If you have something like

struct circle {
   double radius;
   double x,y;
}
struct circle circles[6];

it will occupy 6*3*sizeof(double) bytes in memory (that is usually 144 bytes on 64 bit architecture) in a continuous area. If you have something similar in Java, you need a class (until we get to Java 10 or later):

class Circle {
   double radius;
   double x,y;
}

and the array

Circle circles[6];

will need 6 references (48 bytes or so) and also 6 objects (unless some of them are null) each 24bytes data (or so) and object header (16bytes). That totals to 288bytes on a 64bit architecture and the memory area is not continuous.

When you access an element, say circles[n] of the C language array the code uses pointer arithmetic. It uses the address stored in the pointer circles adds n times sizeof(struct circle) (bytes) and that is where the data is.

The Java approach is a bit different. It looks at the object circles, which is an array, calculates the n-th element (this is similar to C) and fetches the reference data stored there. After the reference data is at hand it uses that to access the object from some different memory location where the reference data leads.

Note that in this case the memory overhead of Java is 100% and also the number of memory reads is 2 instead of 1 to access the actual data.

References do not point to memory

Java references are not pointer. They contain some kind of pointer data or something because that comes from the nature of today computer architecture but this is totally up to the JVM implementation what it stores in a reference value and how it accesses the object it refers to. It could be absolutely ok though not too effective implementation to have a huge array of pointers each pointing to an object of the JVM and the references be indices to this array.

In reality JVM implement the references as a kind of pointer mix, where some of the bits are flags and some of the bits are “pointing” to some memory location relative to some area.

Why do JVMs do that instead of pointers?

The reason is the garbage collection. To implement an effective garbage collection and to avoid the fragmentation of the memory the JVM regularly moves the objects around in the memory. When memory occupied by objects not referenced anymore are freed and we happen to have a small object still used and referenced in the middle of a huge memory block available we do not want that memory block to be split. Instead the JVM moves the object to a different memory area and updates all the references to that object to keep track of the new location. Some GC implementations stop the other Java threads for the time these updates happen, so that no Java code uses a reference not updated but objects moved. Other GC implementations integrate with the underlying OS virtual memory management to cause page fault when such an access occurs to avoid the stopping of the application threads.

However the thing is that references are NOT pointers and it is the responsibility of the implementation of the JVM how it manages all these situations.

The next topic strongly related to this area is parameter passing.

Are parameters passed by value or passed by reference in Java?

The first programming language I studied at the uni was PASCAL invented by Niklaus Wirth. In this language the procedure and function arguments can be passed by value or by reference. When a parameter was passed by reference then the declaration of the argument in the procedure or function head was preceded by the keyword VAR. At the place of the use of the function the programmer is not allowed to write an expression as the actual argument. You have to use a variable and any change to the argument in the function (procedure) will have effect on the variable passed as argument.

When you program in language C you always pass a value. But this is actually a lie, because you may pass the value of a pointer that points to a variable that the function can modify. That is when you write things like char *s as an argument and then the function can alter the character pointed by s or a whole string if it uses pointer arithmetic.

In PASCAL the declaration of pass-by-value OR pass-by-reference is at the declaration of the function (or procedure). In C you explicitly have to write an expression like &s to pass the pointer to the variable s so that the caller can modify it. Of course the function also has to be declared to work with a pointer to a whatever type s has.

When you read PASCAL code you can not tell at the place of the actual function call if the argument is passed-by-value and thus may be modified by the function. In case of C you have to code it at both of the places and whenever you see that the argument value &s is passed you can be sure that the function is capable modifying the value of s.

What is it then with Java? You may program Java for years and may not face the issue or have a thought about it. Java solves the issue automatically? Or just gives a solution that is so simple that the dual pass-by-value/reference approach does not exist?

The sad truth is that Java is actually hides the problem, does not solve it. So long as long we work only with objects Java passes by reference. Whatever expression you write to the actual function call when the result is an object a reference to the object is passed to the method. If the expression is a variable then the reference contained by the variable (which is the value of the variable, so this is a kind of pass-by-value) is passed.

When you pass a primitive (int, boolean etc) then the argument is passed by value. If the expression evaluated results a primitive then it is passed by value. If the expression is a variable then the primitive value contained by the variable is passed. That way we can say looking at the three example languages that

  • PASCAL declares how to pass arguments
  • C calculates the actual value where it is passed
  • Java decides based on the type of the argument

Java, in my opinion, is a bit messy. But I did not realized it because this messiness is limited and is hidden well by the fact that the boxed versions of the primitives are immutable. Why would you care the underlying mechanism of argument passing if the value can not be modified anyway. If it is passed by value: it is OK. If it passed by reference, it is still okay because the object is immutable.

Would it cause problem if the boxed primitive values were mutable? We will see if and when we will have value types in Java.