Examples of com.persistit.Exchange

com.persistit.Exchange
The main facade for fetching, storing and removing records from a Persistit™ database.

Applications interact with Persistit through instances of this class. A Exchange has two important associated member objects, a {@link com.persistit.Key} and a {@link com.persistit.Value}. A Key is a mutable representation of a key, and a Value is a mutable representation of a value. Applications manipulate these objects and interact with the database through one of the following four general patterns:
1. Modify the Key, perform a {@link com.persistit.Exchange#fetch fetch} operation, and query the Value.
2. Modify the Key, modify the Value, and then perform a {@link com.persistit.Exchange#store store} operation to insert or replacedata in the database.
3. Modify the Key, and then perform a {@link com.persistit.Exchange#remove remove} to remove one or more key/valuepairs.
4. Optionally modify the Key, perform a {@link com.persistit.Exchange#traverse traverse} operation, then query theresulting state of Key and/or Value to enumerate key/value pairs currently stored in the database.
Additional methods of Exchange include {@link #fetchAndStore fetchAndStore} and {@link #fetchAndRemove fetchAndRemove} which atomicallymodify the database and return the former value associated with the current Key.

Exchange is Not Threadsafe
Important: an Exchange and its associated Key and Value instances are not thread-safe. Generally each Thread should allocate and use its own Exchange instances. Were it to occur, modification of the Key or Value objects associated with an Exchange by another thread could cause severe and unpredictable errors, including possible corruption of the underlying data storage. While the methods of one Exchange instance are not threadsafe, Persistit is designed to allow multiple threads, using multiple Exchange instances, to access and update the underlying database in a highly concurrent fashion.

Exchange Pools
Normally each thread should allocate its own Exchange instances. However, depending on the garbage collection performance characteristics of a particular JVM it may be desirable to maintain a pool of Exchanges available for reuse, thereby reducing the frequency with which Exchanges need to be constructed and then garbage collected. An application may get an Exchange using {@link Persistit#getExchange(String,String,boolean)} or{@link Persistit#getExchange(Volume,String,boolean)}. These methods reuse a previously constructed Exchange if one is available in a pool; otherwise they construct methods construct a new one. Applications using the Exchange pool should call {@link Persistit#releaseExchange(Exchange,boolean)} to relinquish anExchange once it is no longer needed, thereby placing it in the pool for subsequent reuse.
@version 1.0

            Exchange ex = setupExchange();
            return ex.getKey();
        }


        private Value getValue() throws PersistitException, IOException {
            Exchange ex = setupExchange();
            ex.fetch();
            return ex.getValue();
        }

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            ex.fetch();
            return ex.getValue();
        }


        private String getKeyString() {
            Exchange ex = setupExchange();
            ex.getKey().setIndex(0);
            return ex.getKey().toString();
        }

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            ex.getKey().setIndex(0);
            return ex.getKey().toString();
        }


        private Exchange setupExchange() {
            Exchange ex = PersistitTableModel.this.getExchange();
            Key key = ex.getKey();
            System.arraycopy(_rootKey.getEncodedBytes(), 0, key.getEncodedBytes(), 0, _rootKey.getEncodedSize());
            System.arraycopy(_keyBytes, 0, key.getEncodedBytes(), _rootKey.getEncodedSize(), _keyBytes.length);
            key.setEncodedSize(_rootKey.getEncodedSize() + _keyBytes.length);
            return ex;
        }

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        private boolean isExpanded() {
            return true;
        }


        private Row getNextRow(final boolean forward) throws PersistitException {
            final Exchange ex = setupExchange();
            final boolean expanded = (isExpanded());
            if (!ex.traverse(forward ? Key.GT : Key.LT, expanded)) {
                return null;
            }
            final Key key = ex.getKey();
            if (key.compareKeyFragment(_rootKey, 0, _rootKey.getEncodedSize()) != 0) {
                return null;
            }
            final int keyBytesSize = key.getEncodedSize() - _rootKey.getEncodedSize();
            final byte[] keyBytes = new byte[keyBytesSize];

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            return new Row(keyBytes);
        }


        private String getValueTypeString() {
            try {
                final Exchange ex = setupExchange();
                if (ex.getKey().getEncodedSize() == 0)
                    return "";
                ex.fetch();
                final Value value = ex.getValue();
                if (!value.isDefined())
                    return "undefined";
                return value.getType().getName();
            } catch (final PersistitException de) {
                return de.toString();

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            }
        }


        private String getValueString() {
            try {
                final Exchange ex = setupExchange();
                if (ex.getKey().getEncodedSize() == 0)
                    return "";
                ex.fetch();
                final Value value = ex.getValue();
                if (value.getEncodedSize() > 50000) {
                    return "Too long to display: " + value.getEncodedSize();
                } else {
                    return value.toString();
                }

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                return de.toString();
            }
        }


        private Key getKey() {
            final Exchange ex = setupExchange();
            return ex.getKey();
        }

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            final Exchange ex = setupExchange();
            return ex.getKey();
        }


        private Value getValue() throws PersistitException, IOException {
            final Exchange ex = setupExchange();
            ex.fetch();
            return ex.getValue();
        }

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            ex.fetch();
            return ex.getValue();
        }


        private String getKeyString() {
            final Exchange ex = setupExchange();
            ex.getKey().setIndex(0);
            return ex.getKey().toString();
        }

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            ex.getKey().setIndex(0);
            return ex.getKey().toString();
        }


        private Exchange setupExchange() {
            final Exchange ex = PersistitTableModel.this.getExchange();
            final Key key = ex.getKey();
            System.arraycopy(_rootKey.getEncodedBytes(), 0, key.getEncodedBytes(), 0, _rootKey.getEncodedSize());
            System.arraycopy(_keyBytes, 0, key.getEncodedBytes(), _rootKey.getEncodedSize(), _keyBytes.length);
            key.setEncodedSize(_rootKey.getEncodedSize() + _keyBytes.length);
            return ex;
        }

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Examples of com.persistit.Exchange

Exchange is Not Threadsafe

Exchange Pools

Related Classes of com.persistit.Exchange