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JSON Patch (RFC 6902) and JSON Merge Patch (RFC 7386) implementation in Java
/*
* Copyright (c) 2014, Francis Galiegue ([email protected])
*
* This software is dual-licensed under:
*
* - the Lesser General Public License (LGPL) version 3.0 or, at your option, any
* later version;
* - the Apache Software License (ASL) version 2.0.
*
* The text of both licenses is available under the src/resources/ directory of
* this project (under the names LGPL-3.0.txt and ASL-2.0.txt respectively).
*
* Direct link to the sources:
*
* - LGPL 3.0: https://www.gnu.org/licenses/lgpl-3.0.txt
* - ASL 2.0: http://www.apache.org/licenses/LICENSE-2.0.txt
*/
package com.github.fge.jsonpatch.diff;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.node.ArrayNode;
import com.fasterxml.jackson.databind.node.JsonNodeFactory;
import com.github.fge.jackson.JacksonUtils;
import com.github.fge.jackson.JsonNumEquals;
import com.github.fge.jackson.NodeType;
import com.github.fge.jackson.jsonpointer.JsonPointer;
import com.github.fge.jsonpatch.JsonPatch;
import com.google.common.base.Equivalence;
import com.google.common.collect.Lists;
import java.util.List;
import static com.github.fge.jsonpatch.diff.DiffOperation.*;
/**
* "Reverse" factorizing JSON Patch implementation
*
* This class only has one method, {@link #asJson(JsonNode, JsonNode)}, which
* takes two JSON values as arguments and returns a patch as a {@link JsonNode}.
* This generated patch can then be used in {@link
* JsonPatch#fromJson(JsonNode)}.
*
* Numeric equivalence is respected. Operations are always generated in the
* following order:
*
*
* - additions,
* - removals,
* - replacements.
*
*
* Array values generate operations in the order of elements. Factorizing is
* done to merge add and remove into move operations and convert duplicate add
* to copy operations if values are equivalent. No test operations are
* generated (they don't really make sense for diffs anyway).
*
* Note that due to the way {@link JsonNode} is implemented, this class is
* inherently not thread safe (since {@code JsonNode} is mutable). It is
* therefore the responsibility of the caller to ensure that the calling context
* is safe (by ensuring, for instance, that only the diff operation has
* references to the values to be diff'ed).
*
* @since 1.2
* @author Randy Watler
*/
public final class JsonDiff
{
private static final JsonNodeFactory FACTORY = JacksonUtils.nodeFactory();
private static final Equivalence EQUIVALENCE
= JsonNumEquals.getInstance();
private JsonDiff()
{
}
/**
* Generate a JSON patch for transforming the source node into the target
* node
*
* @param source the node to be patched
* @param target the expected result after applying the patch
* @return the patch as a {@link JsonNode}
*/
public static JsonNode asJson(final JsonNode source, final JsonNode target)
{
// recursively compute node diffs
final List diffs = Lists.newArrayList();
generateDiffs(diffs, JsonPointer.empty(), source, target);
// factorize diffs to optimize patch operations
DiffFactorizer.factorizeDiffs(diffs);
// generate patch operations from node diffs
final ArrayNode patch = FACTORY.arrayNode();
for (final Diff diff: diffs)
patch.add(diff.asJsonPatch());
return patch;
}
/**
* Generate differences between source and target node.
*
* @param diffs list of differences (in order)
* @param path parent path for both nodes
* @param source source node
* @param target target node
*/
private static void generateDiffs(final List diffs,
final JsonPointer path, final JsonNode source, final JsonNode target)
{
/*
* If both nodes are equivalent, there is nothing to do
*/
if (EQUIVALENCE.equivalent(source, target))
return;
/*
* Get both node types. We shortcut to a simple replace operation in the
* following scenarios:
*
* - nodes are not the same type; or
* - they are the same type, but are not containers (ie, they are
* neither objects nor arrays).
*/
final NodeType sourceType = NodeType.getNodeType(source);
final NodeType targetType = NodeType.getNodeType(target);
if (sourceType != targetType || !source.isContainerNode()) {
diffs.add(Diff.simpleDiff(REPLACE, path, target));
return;
}
/*
* At this point, both nodes are either objects or arrays. Call the
* appropriate diff generation methods.
*/
if (sourceType == NodeType.OBJECT)
generateObjectDiffs(diffs, path, source, target);
else // array
generateArrayDiffs(diffs, path, source, target);
}
/**
* Generate differences between two object nodes
*
* Differences are generated in the following order: added members,
* removed members, modified members.
*
* @param diffs list of differences (modified)
* @param path parent path common to both nodes
* @param source node to patch
* @param target node to attain
*/
private static void generateObjectDiffs(final List diffs,
final JsonPointer path, final JsonNode source, final JsonNode target)
{
// compare different objects fieldwise in predictable order;
// maintaining order is cosmetic, but facilitates test construction
final List inFirst = Lists.newArrayList(source.fieldNames());
final List inSecond = Lists.newArrayList(target.fieldNames());
List fields;
// added fields
fields = Lists.newArrayList(inSecond);
fields.removeAll(inFirst);
for (final String s: fields)
diffs.add(Diff.simpleDiff(ADD, path.append(s), target.get(s)));
// removed fields
fields = Lists.newArrayList(inFirst);
fields.removeAll(inSecond);
for (final String s: fields)
diffs.add(Diff.simpleDiff(REMOVE, path.append(s), source.get(s)));
// recursively generate diffs for fields in both objects
fields = Lists.newArrayList(inFirst);
fields.retainAll(inSecond);
for (final String s: fields)
generateDiffs(diffs, path.append(s), source.get(s), target.get(s));
}
/**
* Generate differences between two array nodes.
*
* Differences are generated in order by comparing elements against the
* longest common subsequence of elements in both arrays.
*
* @param diffs list of differences (modified)
* @param path parent pointer of both array nodes
* @param source array node to be patched
* @param target target node after patching
*
* @see LCS#getLCS(JsonNode, JsonNode)
*/
private static void generateArrayDiffs(final List diffs,
final JsonPointer path, final JsonNode source, final JsonNode target)
{
// compare array elements linearly using longest common subsequence
// algorithm applied to the array elements
final IndexedJsonArray src = new IndexedJsonArray(source);
final IndexedJsonArray dst = new IndexedJsonArray(target);
final IndexedJsonArray lcs = LCS.doLCS(source, target);
preLCS(diffs, path, lcs, src, dst);
inLCS(diffs, path, lcs, src, dst);
postLCS(diffs, path, src, dst);
}
/*
* First method entered when computing array diffs. It will exit early if
* the LCS is empty.
*
* If the LCS is not empty, it means that both the source and target arrays
* have at least one element left. In such a situation, this method will run
* until elements extracted from both arrays are equivalent to the first
* element of the LCS.
*/
private static void preLCS(final List diffs, final JsonPointer path,
final IndexedJsonArray lcs, final IndexedJsonArray source,
final IndexedJsonArray target)
{
if (lcs.isEmpty())
return;
/*
* This is our sentinel: if nodes from both the first array and the
* second array are equivalent to this node, we are done.
*/
final JsonNode sentinel = lcs.getElement();
/*
* Those two variables hold nodes for the first and second array in the
* main loop.
*/
JsonNode srcNode;
JsonNode dstNode;
/*
* This records the number of equivalences between the LCS node and
* nodes from the source and target arrays.
*/
int nrEquivalences;
while (true) {
/*
* At each step, we reset the number of equivalences to 0.
*/
nrEquivalences = 0;
srcNode = source.getElement();
dstNode = target.getElement();
if (EQUIVALENCE.equivalent(sentinel, srcNode))
nrEquivalences++;
if (EQUIVALENCE.equivalent(sentinel, dstNode))
nrEquivalences++;
/*
* If both srcNode and dstNode are equivalent to our sentinel, we
* are done; this is our exit condition.
*/
if (nrEquivalences == 2)
return;
/*
* If none of them are equivalent to the LCS node, compute diffs
* in first array so that the element in this array's index be
* transformed into the matching element in the second array; then
* restart the loop.
*
* Note that since we are using an LCS, and no element of either
* array is equivalent to the first element of the LCS (our
* sentinel), a consequence is that indices in both arrays are
* equal. In the path below, we could have equally used the index
* from the target array.
*/
if (nrEquivalences == 0) {
generateDiffs(diffs, path.append(source.getIndex()), srcNode,
dstNode);
source.shift();
target.shift();
continue;
}
/*
* If we reach this point, one array has to catch up in order to
* reach the first element of the LCS. The logic is as follows:
*
* - if the source array has to catch up, it means its elements have
* been removed from the target array;
* - if the target array has to catch up, it means the source
* array's elements are being inserted into the target array.
*/
if (!EQUIVALENCE.equivalent(sentinel, srcNode)) {
diffs.add(Diff.arrayRemove(path, source, target));
source.shift();
} else {
diffs.add(Diff.arrayInsert(path, source, target));
target.shift();
}
}
}
/*
* This method is called after preLCS(). Its role is to deplete the LCS.
*
* One particularity of using LCS is that as long as the LCS is not empty,
* we can be sure that there is at least one element left in both the source
* and target array.
*/
private static void inLCS(final List diffs, final JsonPointer path,
final IndexedJsonArray lcs, final IndexedJsonArray source,
final IndexedJsonArray target)
{
JsonNode sourceNode;
JsonNode targetNode;
JsonNode lcsNode;
boolean sourceMatch;
boolean targetMatch;
while (!lcs.isEmpty()) {
sourceNode = source.getElement();
targetNode = target.getElement();
lcsNode = lcs.getElement();
sourceMatch = EQUIVALENCE.equivalent(sourceNode, lcsNode);
targetMatch = EQUIVALENCE.equivalent(targetNode, lcsNode);
if (!sourceMatch) {
/*
* At this point, the first element of our source array has
* failed to "reach" a matching element in the target array.
*
* Such an element therefore needs to be removed from the target
* array. We therefore generate a "remove event", shift the
* source array and restart the loop.
*/
diffs.add(Diff.arrayRemove(path, source, target));
source.shift();
continue;
}
/*
* When we reach this point, we know that the element extracted
* from the source array is equivalent to the LCS element.
*
* Note that from this point on, whatever the target element is, we
* need to shift our target array; there are two different scenarios
* we must account for:
*
* - if the target element is equivalent to the LCS element, we have
* a common subsequence element (remember that the source element
* is also equivalent to this same LCS element at this point); no
* mutation of the target array takes place; we must therefore
* shift all three arrays (source, target, LCS);
* - otherwise (target element is not equivalent to the LCS
* element), we need to emit an insertion event of the target
* element, and advance the target array only.
*/
if (targetMatch) {
source.shift();
lcs.shift();
} else
diffs.add(Diff.arrayInsert(path, source, target));
/*
* Shift/advance the target array; always performed, see above
*/
target.shift();
}
}
/*
* This function is run once the LCS has been exhausted.
*
* Since the LCS has been exhausted, it means that for whatever nodes node1
* and node2 extracted from source and target, they can never be equal.
*
* The algorithm is therefore as follows:
*
* - as long as both are not empty, grab both elements from both arrays and
* generate diff operations on them recursively;
* - when we are out of this loop, add any elements remaining in the second
* array (if any), and remove any elements remaining in the first array
* (if any).
*
* Note that at the second step, only one of the two input arrays will ever
* have any elements left; it is therefore safe to call the appropriate
* functions for _both_ possibilities since only one will ever produce any
* results.
*/
private static void postLCS(final List diffs, final JsonPointer path,
final IndexedJsonArray source, final IndexedJsonArray target)
{
JsonNode src, dst;
while (!(source.isEmpty() || target.isEmpty())) {
src = source.getElement();
dst = target.getElement();
generateDiffs(diffs, path.append(source.getIndex()), src, dst);
source.shift();
target.shift();
}
addRemaining(diffs, path, target);
removeRemaining(diffs, path, source);
}
private static void addRemaining(final List diffs,
final JsonPointer path, final IndexedJsonArray array)
{
Diff diff;
JsonNode node;
while (!array.isEmpty()) {
node = array.getElement().deepCopy();
diff = Diff.arrayAdd(path, node);
diffs.add(diff);
array.shift();
}
}
private static void removeRemaining(final List diffs,
final JsonPointer path, final IndexedJsonArray array)
{
final int startingIndex = array.getIndex();
Diff diff;
JsonNode node;
while (!array.isEmpty()) {
node = array.getElement();
diff = Diff.tailArrayRemove(path, startingIndex, node);
diffs.add(diff);
array.shift();
}
}
}
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