Encoder is a class which can be used to create
* files or streams that encode the state of a collection of
* JavaBeans in terms of their public APIs. The Encoder,
* in conjunction with its persistence delegates, is responsible for
* breaking the object graph down into a series of Statementss
* and Expressions which can be used to create it.
* A subclass typically provides a syntax for these expressions
* using some human readable form - like Java source code or XML.
*
* @since 1.4
*
* @author Philip Milne
*/
public class Encoder {
private final PersistenceDelegateFinder finder = new PersistenceDelegateFinder();
private Map bindings = new IdentityHashMap();
private ExceptionListener exceptionListener;
boolean executeStatements = true;
private Map attributes;
/**
* Write the specified object to the output stream.
* The serialized form will denote a series of
* expressions, the combined effect of which will create
* an equivalent object when the input stream is read.
* By default, the object is assumed to be a JavaBean
* with a nullary constructor, whose state is defined by
* the matching pairs of "setter" and "getter" methods
* returned by the Introspector.
*
* @param o The object to be written to the stream.
*
* @see XMLDecoder#readObject
*/
protected void writeObject(Object o) {
if (o == this) {
return;
}
PersistenceDelegate info = getPersistenceDelegate(o == null ? null : o.getClass());
info.writeObject(o, this);
}
/**
* Sets the exception handler for this stream to exceptionListener.
* The exception handler is notified when this stream catches recoverable
* exceptions.
*
* @param exceptionListener The exception handler for this stream;
* if null the default exception listener will be used.
*
* @see #getExceptionListener
*/
public void setExceptionListener(ExceptionListener exceptionListener) {
this.exceptionListener = exceptionListener;
}
/**
* Gets the exception handler for this stream.
*
* @return The exception handler for this stream;
* Will return the default exception listener if this has not explicitly been set.
*
* @see #setExceptionListener
*/
public ExceptionListener getExceptionListener() {
return (exceptionListener != null) ? exceptionListener : Statement.defaultExceptionListener;
}
Object getValue(Expression exp) {
try {
return (exp == null) ? null : exp.getValue();
}
catch (Exception e) {
getExceptionListener().exceptionThrown(e);
throw new RuntimeException("failed to evaluate: " + exp.toString());
}
}
/**
* Returns the persistence delegate for the given type.
* The persistence delegate is calculated
* by applying the following of rules in order:
* BeanDescriptor
* which defined a "persistenceDelegate" property, this
* value is returned.
*
* public class Foo {
* public Foo() { ... }
* public Foo(int x) { ... }
* }
* public class Bar {
* public Bar() { ... }
* @ConstructorProperties({"x"})
* public Bar(int x) { ... }
* @ConstructorProperties({"x", "y"})
* public Bar(int x, int y) { ... }
* }
*
* type to
* persistenceDelegate.
*
* @param type The class of objects that persistenceDelegate applies to.
* @param persistenceDelegate The persistence delegate for instances of type.
*
* @see #getPersistenceDelegate
* @see java.beans.Introspector#getBeanInfo
* @see java.beans.BeanInfo#getBeanDescriptor
*/
public void setPersistenceDelegate(Class type,
PersistenceDelegate persistenceDelegate)
{
synchronized (this.finder) {
this.finder.register(type, persistenceDelegate);
}
}
/**
* Removes the entry for this instance, returning the old entry.
*
* @param oldInstance The entry that should be removed.
* @return The entry that was removed.
*
* @see #get
*/
public Object remove(Object oldInstance) {
Expression exp = (Expression)bindings.remove(oldInstance);
return getValue(exp);
}
/**
* Returns a tentative value for oldInstance in
* the environment created by this stream. A persistence
* delegate can use its mutatesTo method to
* determine whether this value may be initialized to
* form the equivalent object at the output or whether
* a new object must be instantiated afresh. If the
* stream has not yet seen this value, null is returned.
*
* @param oldInstance The instance to be looked up.
* @return The object, null if the object has not been seen before.
*/
public Object get(Object oldInstance) {
if (oldInstance == null || oldInstance == this ||
oldInstance.getClass() == String.class) {
return oldInstance;
}
Expression exp = (Expression)bindings.get(oldInstance);
return getValue(exp);
}
private Object writeObject1(Object oldInstance) {
Object o = get(oldInstance);
if (o == null) {
writeObject(oldInstance);
o = get(oldInstance);
}
return o;
}
private Statement cloneStatement(Statement oldExp) {
Object oldTarget = oldExp.getTarget();
Object newTarget = writeObject1(oldTarget);
Object[] oldArgs = oldExp.getArguments();
Object[] newArgs = new Object[oldArgs.length];
for (int i = 0; i < oldArgs.length; i++) {
newArgs[i] = writeObject1(oldArgs[i]);
}
Statement newExp = Statement.class.equals(oldExp.getClass())
? new Statement(newTarget, oldExp.getMethodName(), newArgs)
: new Expression(newTarget, oldExp.getMethodName(), newArgs);
newExp.loader = oldExp.loader;
return newExp;
}
/**
* Writes statement oldStm to the stream.
* The oldStm should be written entirely
* in terms of the callers environment, i.e. the
* target and all arguments should be part of the
* object graph being written. These expressions
* represent a series of "what happened" expressions
* which tell the output stream how to produce an
* object graph like the original.
*
* The implementation of this method will produce
* a second expression to represent the same expression in
* an environment that will exist when the stream is read.
* This is achieved simply by calling writeObject
* on the target and all the arguments and building a new
* expression with the results.
*
* @param oldStm The expression to be written to the stream.
*/
public void writeStatement(Statement oldStm) {
// System.out.println("writeStatement: " + oldExp);
Statement newStm = cloneStatement(oldStm);
if (oldStm.getTarget() != this && executeStatements) {
try {
newStm.execute();
} catch (Exception e) {
getExceptionListener().exceptionThrown(new Exception("Encoder: discarding statement "
+ newStm, e));
}
}
}
/**
* The implementation first checks to see if an
* expression with this value has already been written.
* If not, the expression is cloned, using
* the same procedure as writeStatement,
* and the value of this expression is reconciled
* with the value of the cloned expression
* by calling writeObject.
*
* @param oldExp The expression to be written to the stream.
*/
public void writeExpression(Expression oldExp) {
// System.out.println("Encoder::writeExpression: " + oldExp);
Object oldValue = getValue(oldExp);
if (get(oldValue) != null) {
return;
}
bindings.put(oldValue, (Expression)cloneStatement(oldExp));
writeObject(oldValue);
}
void clear() {
bindings.clear();
}
// Package private method for setting an attributes table for the encoder
void setAttribute(Object key, Object value) {
if (attributes == null) {
attributes = new HashMap();
}
attributes.put(key, value);
}
Object getAttribute(Object key) {
if (attributes == null) {
return null;
}
return attributes.get(key);
}
}