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Cover image for Effective Java β€”Chapters 10 to 12
Ashkan Entezari
Ashkan Entezari

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Effective Java β€”Chapters 10 to 12

Introduction

This the final part of the summary of this book. In part one I summarized chapters 2 to 5 and in part two I summarized chapters 6 to 9. In this final part, we will have the summary of remainder of the book, chapters 10 to 12. If you found issues with any part of the summary or if you have suggestions to make them better or more understandable, please let me know and I will be updating them.

10 Exceptions

Item 69

  • Use exceptions only for exceptional conditions
  • we should not use them for ordinary conditional flow like this:
try {
    int i = 0;
    while (true) {
        range[i++].climb();
    }
} catch (ArrayIndexOutOfBoundsException e) {}
  • this approach of using exceptions is wrong on different levels:
    • it is confusing as it's not the intention of the exceptions
    • JVM will not optimize these (compared to when we use Arrays.length() for instance)
    • we may catch an actual exception here unintentionally without realizing it
  • a well designed API must not force its clients to use exceptions for ordinary workflow
    • e.g. Iterator has hasNext() which you can use before calling next(), so you don't need try/catch here
    • in general, when a method is state-dependant, we should have a state-testing method to call before calling our method to make sure we can safely call it.

Item 70

  • Use checked exceptions for recoverable conditions and runtime exceptions for programming errors
  • Java provides three kinds of throwables:
    1. checked exceptions
    2. when we declare a method throws an exception
    3. the caller needs to catch it and resolve it or propagate it upwards
    4. they are meant to be used in conditions from which the caller can reasonably be expected to recover
    5. it is a bad idea to just catch these exceptions and ignore them.
    6. provide methods on checked exceptions to aid in recovery. e.g. we have a payment w/ credit card method that throws exception if funds are not sufficient. We can provide an accessor method to query the amount of shortfall. This enables the caller to relay the amount to the shopper
    7. runtime exceptions
    8. these exceptions are usually result of precondition violation which is client failure to adhere to the contract established by the API specifications
    9. they are result of programming errors
    10. e.g. the contract for array access specifies the indices to be between 0 and array's length minus 1, using something outside this range will throw ArrayIndexOutOfBoundsException
    11. errors
    12. there is a strong convention that errors are reserved for use by the JVM
    13. such as OutOfMemoryError, ThreadDeath, etc.
    14. all the unchecked exceptions you implement should subclass RuntimeException

Item 71

  • Avoid unnecessary use of checked exceptions
  • when used sparingly they increase reliability of programs, when overused, they make APIs painful to use
  • we need to use them when we believe user can recover the situation when getting the exception
    • if they just log the stack trace or ignore it (as in like they can do nothing else) it doesn't make sense to make it a checked exception. This just makes it harder to use as you have to use a try/catch or propagate it outward!

Item 72

  • Favor the use of standard exceptions
  • Java libraries provide a set of exceptions that covers most of exception throwing needs of most APIs
  • we should try to use standard and appropriate exception types which makes our code cleaner and easier to read and understand
  • the most commonly reused exceptions are:
    • IllegalArgumentException: non-null parameter value is inappropriate
    • like passing a negative number to an argument expecting number of elements
    • IllegalStateException: Object state is inappropriate for method invocation
    • like Object is not initialized
    • NullPointerException: parameter value is null where prohibited
    • IndexOutOfBoundException: index parameter value is out of range
    • ConcurrentModificationException: concurrent modification of object has been detected where it is prohibited
    • UnsupportedOperationException: object does not support method

Item 73

  • Throw exceptions appropriate to the abstraction
  • when exceptions propagate outward, we may get one that has no apparent connection to the task that it performs.
  • to avoid this problem we can do exception translation which is basically catching the exception (when appropriate) and throwing a more relevant exception to the higher level instead
  • this is an example from AbstractSequentialList:
public E get(int index) {
    ListIterator<E> i = listIterator(index);
    try {
        return i.next();
    } catch (NoSuchElementException e) {
        throw new IndexOutOfBoundsException("Index: " + index);
    }
}
  • a special form of exception translation is called exception chaining which is when lower level exception might be useful to someone debugging the higher level exception. in this case, we pass the lower level exception to the higher level exception:
try {
    ...
} catch (LowerLevelException cause) {
    throw new HigherLevelException(cause);
}
  • most standard exceptions have chaining-aware constructors which pass the "cause" to a higher level constructor which can be accessed later.
  • for exceptions, we can use Throwable's initCause method which later can be accessed using getCause

Notes

  • while exception translation is superior to mindless propagation of exceptions from lower layers, it should not be overused

Item 74

  • Document all exceptions thrown by each method
  • always declare checked exceptions individually and document precisely the conditions under which each one is thrown
  • use JavaDoc's @throws tag
  • it is also a good practice to document unchecked exceptions that the method can throw. These unchecked exceptions are usually programmer's error and doing this, they will know what to expect and how to properly use that method or interface.

Item 75

  • Include failure-capture information in detail messages
  • to capture a failure, the detail message of an exception should contain all the values for all the parameters and fields that contributed to the exception
    • e.g. if we get IndexOutOfBoundException, it would be very helpful to have the index value as well as the lower bound and upper bound
    • an exception to this (for security purposes) is passwords, encryption keys and the like which we don't wanna include in the detail message

Item 76

  • Strive for failure atomicity
  • it means a failed method invocation should leave the object in the state that it was in prior to the invocation
  • there are different ways to achieve this goal. For example in stack implementation from before:
public Object pop() {
    if (size == 0) {
        throw new EmptyStackException();
    }
    Object result = elements[--size];
    ...
}

so here we check the size first and throw exception instead of making size -1 and then wait for the exception to be thrown when accessing elements[-1]

  • another way to accomplish failure atomicity is to change the order of the computations if possible, so the part that may cause exception comes first.
  • Another approach is to perform operations on a copy of the object and then if everything was successful apply them back to the original
  • it should be mentioned that failure atomicity is not always achievable (like to threads without proper synchronization modify something) or sometimes making something atomic will hugely complicate the code. In these cases we should mention it in the API documentation that what is the broken state that may happen by a method invocation for example

Item 77

  • Don't ignore exceptions
  • an empty catch block defeats the purpose of exceptions. We should always take proper actions to address exceptions
  • in some rare cases it might be ok to ignore them, in which case we should put a comment in catch block with explanations:
int numColors = 4; // Default
try {
    numColors = getNumColors();
} catch (ExecutionException | TimedoutException ignored) {
    // in this case just use default value
}

11 Concurrency

Item 78

  • Synchronize access to shared mutable data
  • it is better not to share mutable data between multiple threads and share only immutable data.
  • if we have a mutable object, we can share it with only one thread
  • if we have to share mutable data, we should use Synchronize which guarantees that no method will ever observe the object in an inconsistent state.
    • the language specification guarantees that read/write is atomic unless variable is long or double (there is AtomicLong that we can use)
  • one example of using synchronization:
public class StopThread() {
    private static boolean stopRequested;

    private static synchronized void requestStop() {
        stopRequested = true;
    }

    private static synchronized boolean stopRequested() {
        return stopRequested;
    }

    public static void main(String[] args) {
        Thread backgroundThread = new Thread(() -> {
            int i = 0;
            while ( !stopRequested() ) {
                i++;
            }
        });
        backgroundThread.start();

        TimeUnit.SECONDS.sleep(1);
        requestStop();
    }
}
  • synchronization can be used for:
    • 'communication': if we remove synchronization code above, our thread won't have access to updated value of stopRequested, so we should use it to make sure it does get the updated value as we expect it
    • 'mutual exclusion': so we won't have race condition or having one thread reading value while the other is writing a new value to it.
  • in the above example, since synchronization is only used fo 'communication', we can remove the synchronized methods for read and write and instead declare the variable like this: private static volatile boolean stopRequested; this will make the thread check for the updated value every time it reads it.
  • synchronization is not guaranteed unless both read/write methods are synchronized.

Item 79

  • Avoid excessive synchronization
  • inside a synchronized method or block, never cede control to the client
    • like calling a method that is designed to be overridden
    • or calling a method provided by the client
    • from the perspective of the class with the synchronized region, these methods are called alien
  • we may get unwanted exceptions or deadlocks
  • as a rule, we should do as little work as possible inside synchronized regions
  • over-synchronization can have its own issues

Item 80

  • Prefer executors, tasks, and streams to threads
  • java.util.concurrent package contains Executor Framework which is a flexible interface-based task execution facility. Instead of creating a work queue to execute tasks asynchronously and handling that queue manually, it is better to use this framework:
ExecutorService exec = Executors.newSingleThreadExecutor();
// submitting a runnable for execution:
exec.execute(runnable);
// terminating executor gracefully:
exec.shutdown();
  • you can do many more with this service. You can wait for all the tasks or a particular task to complete, you can retrieve results of tasks as they complete, you can schedule them to run at a particular time or to run periodically, and so on.
  • in essence, Executor Framework does for execution what Collections Framework did for aggregation.

Item 81

  • Prefer concurrency utilities to wait and notify
  • the higher level utilities in java.util.concurrent fall into three categories:
    1. Executor Framework
    2. concurrent collections
    3. synchronizers
  • concurrent collections are high-performance concurrent implementation of standard collection interfaces such as List, Queue and Map (like ConcurrentHashMap)
  • Synchronizers are objects that enable threads to wait for one another, allowing them to coordinate their activities. the most common ones are CountDownLatch and Semaphore and the most powerful one is Phaser.
  • quick explanation of CountDownLatch: it has a constructor that gets an int which is number of times the countDown method must be invoked on it before all waiting threads can proceed:
CountDownLatch latch = new CountDownLatch(2);
latch.await();
System.out.println("done!");

so here if in another thread for example we invoke latch.countDown two times, the above code will proceed and print "done!"

  • in summary, this item (and the previous one) introduced us to three main utilities of java.util.concurrent. There are old alternatives for each of them that require lots of code and maintenance (for #2 above, there are Synchronized Collections that are slower. For #3 there is wait and notify/notifyAll which is messy and needs more handling). These last two items introduce these new alternatives and encourage us to use them instead. There are lots of details for each of them and if we need them, we should study them in more depth.

Item 82

  • Document thread safety
  • How a class behaves when its methods are called concurrently is an important part of its API contract
  • to enable safe concurrent use, a class must clearly document its level of thread safety:
    • Immutable: Instances are constant, no external synchronization is necessary
    • Unconditionally thread-safe: instances of this class are mutable but the class has sufficient internal synchronization
    • Conditionally thread-safe: like previous one but some of its methods need external synchronization
    • Not thread-safe: to use them safely, client must surround each method invocation with external synchronization
    • Thread hostile: unsafe for concurrent use even if everything is surrounded by synchronization. This usually results from modifying static data without internal synchronization, external synchronization won't help here!
  • lock fields should always be declared final:
    private final Object lock = new Object();

    public void foo() {
        synchronized(lock) {
            ...
        }
    }

Item 83

  • Use lazy initialization judiciously
  • Lazy initialization: the act of delaying the initialization of a field until its value is needed
  • as is the case for most optimizations, don't do it unless you need to
  • if a field is only accessed on a fraction of the instances and it's costly to initialize it, it's better to lazily initialize it

  • normal lazy initialization which needs to be synchronized:

private FieldType field;
private synchronized FieldType getField() {
    if (field == null) {
        field = computeFieldValue();
    }
    return field;
}
  • lazy initialization holder class idiom, to use on a static field:
private static class FieldHolder {
    static final FieldType field = computeFieldValue(); // this will be ignored until accessor is called
}
private static FieldType getField() {
    return FieldHolder.field;
}
  • to use lazy initialization for performance on an instance field, use the double-check idiom:
    private volatile FieldType field;

    private FieldType getField() {
        FieldType result = field;
        if (result == null) { // first check, no locking
            synchronized(this) {
                if (field == null) { // second check with locking
                    field = result = computeFieldValue();
                }
            }
        }
    }

Item 84

  • Don't depend on the thread scheduler
  • when there are many runnable threads at the same time, thread scheduler determines which one gets to run for how long and this can be different in different systems. Any program that relies on thread scheduler for correctness and performance is likely to be nonportable.
  • threads should not run if they are not doing useful work (like waiting for something)
  • resist the temptation to use Thread.yield or thread priorities which are among least portable features of Java

12 Serialization

Item 85

  • Prefer alternatives to Java serialization
  • There are many security risks in serialization. the best way to avoid serialization exploits is to never deserialize anything
  • there is no reason to use Java serialization in any new code you write
  • two good alternatives is to use JSON or protobuf as a cross platform structured data representation

Item 86

  • Implement Serializable with a great caution
  • you can simply make any class to implement Serializable but there are a few things to be aware of
    • it decreases class's flexibility to change once it has been released. basically the byte stream of its internals are now part of the exported API
    • it increases the likelihood of bugs and security holes
    • it increases the testing burden. It should be possible to serialize the old version of a class and deserialize it using its new version and vice versa. These should be all tested
    • Classes designed for inheritance should rarely implement Serializable and interfaces should rarely extend it
    • inner classes should not implement Serializable
  • when you make a serializable class, if you don't declare a static final long field called serialVersionUID, the system automatically generates it at runtime

Item 87

  • Consider using a customer serialized form
  • Do not accept default serialized form without considering whether it is appropriate or not.
  • default serialization is an efficient encoding of the physical representation of the object graph
  • if this physical representation is identical to its logical content, then it is appropriate to use the default serialization. The following class is an example:
public class Name implements Serialization {
    /**
    * Last name, must be non-Null
    * @serial
    */
    private final String lastName;

    /**
    * First name, must be non-Null
    * @serial
    */
    private final String firstName;

    /**
    * Middle name, or null if there is none
    * @serial
    */
    private final String middleName;
}
  • even if you decide that the default serialized form is appropriate, you often must provide a readObject method
  • On the other end of the spectrum, here is a terrible candidate for default serialization:
public final class StringList implements Serializable {
    private int size = p;
    private Entry head = null;

    private static class Entry implements Serializable {
        String data;
        Entry next;
        Entry previous;
    }
    //...
}

logically speaking, this class represents a sequence of strings but physically it is a sequence of doubly linked list. A reasonable serialization form of this class is simply a number, representing number of strings in the list, followed by the strings themselves:

public final class StringList implements Serializable {
    private transient int size = 0;
    private transient Entry head = null;

    private static class Entry {
        String data;
        Entry next;
        Entry previous;
    }

    // appends the specified string to the list
    public final void add(String s) {...}

    /**
    * Serialize this {@code StringList} instance
    *
    * @serialData The size of the list (the number of strings it contains) is
    * emitted ({@code int}), followed by all of its elements (each a {@code String}
    * ), in the proper sequence
    */
    private void writeObject(ObjectOutputStream s) throws IOException {
        s.defaultWriteObject();
        s.writeInt(size);

        // write out all elements in the proper order
        for (Entry e = head; e != null; e = e.next) {
            s.writeObject(e.data);
        }
    }

    private void readObject(ObjectInputStream s) throws IOException {
        s.defaultReadObject();
        int numElements = s.readInt();

        // read in all elements and insert them in list
        for (int i = 0; i < numElements; i++) {
            add((String) s.readObject());
        }
    }

    // remainder omitted
}

Item 88

  • Write readObject methods defensively
  • default serialization and deserialization of classes can be overridden using readObject and writeObject methods.
  • these can be used for different scenarios, for example initializing transient (non-serializable) fields once we deserialized the object
  • another use case which is the topic of this item is protecting immutable classes when we deserialize them
  • as a reminder, for classes that have modifiable objects, we make them immutable by making them private final and also defensively copying them everywhere:
public final class Period {
    private final Date start;
    private final Date end;

    public Period(Date start, Date end) {
        this.start = new Date(start.getTime());
        this.end = new Date(end.getTime());
        if (this.start.compareTo(this.end) > 0) {
            throw new IllegalArgumentException();
        }
    }

    // getters and no setters
}

other than defensive copying, we check for validations as well to make sure state is valid.

  • This item explains that readObject is effectively another public constructor which demands all of the same care as any other constructor
    • there are a few examples in the book that modifies the serialized byte stream to break things that are normally not possible on the above Period class. (examples are for when we make Period to implement Serializable and we don't provide a proper readObject)
    • in short, the proper readObject should defensively copy the fields and check for the validations
    • here is a proper implementation of it:
  private void readObject(ObjectInputStream is) throws IOException, ClassNotFoundException {
      is.defaultReadObject();

      start = new Date(start.getTime());
      end = new Date(end.getTime());

      if (start.compareTo(end) > 0) {
          throw new InvalidObjectException();
      }
  }

note that here we have to make the fields non-final to be able to make a defensive copy right after deserialization.

  • same as immutable classes, these readObject methods should not invoke overridable methods

Item 89

  • For instance control, prefer enum types to readResolve
  • another risk with using serialization is singleton classes. They look like this:
public class Elvis {
    public static final Elvis INSTANCE = new Elvis();
    private Elvis() {...}
}

if we make it to implement serializable, it no longer will be a singleton.

  • As a fix we can use readResolve method. This method takes object after deserialization and the object returned by this method is the final deserialized object (so we can replace the serialized object or modify it however we want before it gets to the client). The fix using readResolve will look something like this:
private Object readResolve() {
    // return the only instance of Elvis and garbage collector will take care of
    // the deserialized Elvis impersonator which will be ignored here:
    return INSTANCE;
}
  • if we depend on readResolve for instance control, all instance fields with object reference types must be declared transient, if not there can be a potential for an attack.
  • We can prevent this attack with a more preferred approach which is using enum:
public enum Elvis {
    INSTANCE;

    private String[] favoriteSongs = {"a", "b"};

    public void printFavourites() {
        System.out.println(Arrays.toString(favoriteSongs));
    }
}

Item 90

  • Consider serialization proxies instead of serialized instances
  • consider this pattern whenever you find yourself having to write readObject or writeObject on a class that is not extendable by its clients
  • as mentioned in previous items, using serialization increases the likelihood of bugs and security problems. Serialization Proxy Pattern greatly reduces these risks
  • first, design a private static nested class that concisely represents the logical state of an instance of the enclosing class (this nested class is serialization proxy of the enclosing class)
  • this class should have one constructor whose parameter type is enclosing class.
  • both enclosing class and serialization proxy must declare Serialization
  • this is a serialization proxy for the Period class written in item 50:
private static class SerializationProxy implements Serializable {
    private final Date start;
    private final Date end;

    SerializationProxy(Period period) {
        this.start = period.start;
        this.end = period.end;
    }

    private static final long serialVersionUID = 23413487423847742L; // any number will do
}
  • now we have to add the writeReplace method to the enclosing class:
private Object writeReplace() {
    return new SerializationProxy(this);
}
  • with this pattern, we should not generate a serialized version of the enclosing class, to safe gaurd it against attackers, add this to the enclosing class:
private void readObject(ObjectInputStream stream) throws InvalidObjectException {
    throw new InvalidObjectException("Proxy required!");
}
  • finally, provide readResolve method on the serialization proxy class that returns a logically equivalent instance of the enclosing class:
private Object readResolve() {
    return new Period(start, end);
}
  • the key here is that at the end, this pattern uses the public constructor in the readResolve method which makes it safe (normally serialization uses extralinguistic mechanism in place of ordinary constructors)

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