Example 9.18, “GML representation” shows the GML representation for the example graph from Figure 9.4, “A simple graph”.

The GML format in its entirety is exhaustively presented in the section called “GML File Format”.

Figure 9.4. A simple graph

Creator	"yFiles"
Version	2.2
graph
[ hierarchic  1
  directed  1
  node
  [ id  0
    graphics
    [ x 200.0	
      y 0.0
    ]
    LabelGraphics
    [ text  "January" ]
  ]
  node
  [ id  1
    graphics
    [ x 425.0
      y 75.0
    ]
    LabelGraphics
    [ text  "December"  ]
  ]
  edge
  [ source  1
    target  0
    graphics
    [ Line
      [ point
        [ x 425.0
          y 75.0
        ]
        point
        [ x 425.0
          y 0.0
        ]
        point
        [ x 200.0
          y 0.0
        ]
      ]
    ]
    LabelGraphics
    [ text  "Happy New Year!"
      model "six_pos"
      position  "head"
    ]
  ]
]

A GML file is written using class GMLIOHandler. The code fragment in Example 9.19, “Instantiating a GMLIOHandler” shows how to instantiate a GMLIOHandler and using it to write a graph to file.

// Instantiate a GML I/O handler and write the graph to file. 
IOHandler ioh = new GMLIOHandler();
writeGraphToFile(graph, ioh, "MyGML.gml");

Adding custom data from a graph to a GML file can be accomplished by providing appropriate encoder and parser logic. In package y.io.gml reside the classes for the default GML encoding and parsing behavior. These classes cover all sorts of graph elements, i.e., nodes and edges, but also hierarchy nodes and inter-edges. For any of these elements there are classes available for both input and output, i.e., to encode the respective item and to parse it.

Replacing the default GML I/O logic with customized behavior takes place at the level of these classes. Due to the nature of both the GML encoding as well as its parsing process, however, there have to be provided additional classes, so-called factories. These classes are responsible for instantiating the customized classes each time the encoding or parsing process needs the respective functionality.

The factories have to be properly registered with the GMLIOHandler instance that is used for file I/O. This is done using the following methods from class GMLIOHandler. Accordingly, the registered factories implement interfaces EncoderFactory and ParserFactory, respectively.

static void setEncoderFactory(EncoderFactory _encoderFactory)
static void setParserFactory(ParserFactory _parserFactory)

The following tables describe all GML features, the attributes, and keys/tags that are handled by class GMLIOHandler.

Basic GML

The base set of GML tags. These are essential to save the structure of a graph.

Table 9.5. GML toplevel

Item	Type	Description	Parsed	Written
.Creator	attribute [String]	The name of the creator of the file.	No	Yes
.Version	attribute [Double]	The version of the creator.	No	Yes
.graph	section (single)	The actual graph data.	Yes	Yes

Table 9.6. GML .graph level

Item	Type	Description	Parsed	Written
.label	attribute [String]	The textual name of the graph.	No	Yes
.directed	attribute [Integer / Boolean]	Predicate whether edges are directed.	No	Yes
.node	section (multiple)	A simple node.	Yes	Yes
.edge	section (multiple)	A simple edge.	Yes	Yes
.defaultnodesize	attribute [String]	May be set to 'labelsize'. The parser will automatically assign a size for nodes where width or height is not set.	Yes	No
.defaultnodeinsetshorizontal	attribute [Integer]	Used in conjunction with defaultnodesize = 'labelsize'. Determines horizontal insets between label and node border.	Yes	No
.defaultnodeinsetsvertical	attribute [Integer]	Used in conjunction with defaultnodesize = 'labelsize'. Determines vertical insets between label and node border.	Yes	No
.defaultnodeminwidth	attribute [Integer]	Used in conjunction with defaultnodesize = 'labelsize'. Determines minimal width of automatically assigned widths.	Yes	No
.defaultnodeminheight	attribute [Integer]	Used in conjunction with defaultnodesize = 'labelsize'. Determines minimal height of automatically assigned heights.	Yes	No

Table 9.7. GML .graph.node level

Item	Type	Description	Parsed	Written
.id	attribute [Integer]	The unique node ID.	Yes	Yes
.name	attribute [String]	The textual name of the node (label).	Yes	No
.label	attribute [String]	The textual name of the node (label).	Yes	Yes
.graphics	section (single)	The graphical representation of the node.	Yes	Yes
.LabelGraphics	section (multiple)	Label graphics definitions.	Yes	Yes

Table 9.8. GML .graph.node.graphics level

Item	Type	Description	Parsed	Written
.x	attribute [Double]	The x-coordinate of the center of the node.	Yes	Yes
.y	attribute [Double]	The y-coordinate of the center of the node.	Yes	Yes
.w	attribute [Double]	The width of the node.	Yes	Yes
.h	attribute [Double]	The height of the node.	Yes	Yes
.type	attribute [String]	The name of the shape type. This is one of: "ellipse"/"oval"/"circle", "rectangle", "triangle", "parallelogram", "hexagon", "diamond", "octagon", "roundrectangle", "rectangle3d", "trapezoid", "trapezoid2". "ellipse" / "oval" / "circle" "rectangle" "triangle" "parallelogram" "hexagon" "diamond" "octagon" "roundrectangle" "rectangle3d" "trapezoid" "trapezoid2"	Yes	Yes
.dropShadowColor	attribute [Color]	The color of a node's drop shadow.	Yes	Yes
.dropShadowOffsetX	attribute [Double]	The drop shadow's x-offset.	Yes	Yes
.dropShadowOffsetY	attribute [Double]	The drop shadow's y-offset.	Yes	Yes
.image	attribute [String]	The URL of the image used for image nodes.	Yes	Yes
.alphaImage	attribute [Integer / Boolean]	Whether or not alpha transparency is supported to modify an image node's hit-testing behavior.	Yes	Yes
.customconfiguration	attribute [String]	The configuration of a generic node.	Yes	Yes
.userdataclass	attribute [String]	The class name for the user data part of a generic node.	Yes	Yes
.userdata	attribute [String]	The user data part of a generic node.	Yes	Yes
.fill	attribute [Color]	The fill color of the node.	Yes	Yes
.hasFill	attribute [Integer / Boolean]	Whether or not the node has a fill color.	Yes	Yes
.fill2	attribute [Color]	A second fill color for a node.	Yes	Yes
.outline	attribute [Color]	The stroke color used for the node.	Yes	Yes
.hasOutline	attribute [Integer / Boolean]	Whether or not the node has an outline.	Yes	Yes
.width	attribute [Integer]	The line width of the outline.	Yes	No
.outlineWidth	attribute [Integer]	The line width of the outline.	Yes	Yes
.outlineStyle	attribute [String]	The name of the outline style. This is one of: "line", "dashed", "dotted".	Yes	Yes

Table 9.9. GML .graph.node.LabelGraphics level

Item	Type	Description	Parsed	Written
.type	attribute [String]	The content type of the label, either "text" for textual labels or "index_label" for automatic numeric labels.	Yes	No
.text	attribute [String]	The label's text.	Yes	Yes
.outline	attribute [Color]	The outline color of the label.	Yes	Yes
.fill	attribute [Color]	The background color of the label.	Yes	Yes
.color	attribute [Color]	The text color of the label.	Yes	Yes
.model	attribute [String]	The label model. Only written when the label model is not internal, and does not contain the entire set of positions outside the node.	Yes	Yes
.anchor	attribute [String]	The anchor type of the label that determines the position of the label relative to its node. Positions within the node are "c", "t", "tl", "tr", "l", "r", "bl", "b", or "br". Positions around the node are "n", "ne", "e", "se", "s", "sw", "w", or "nw".	Yes	Yes
.x	attribute [Double]	The absolute x-coordinate of the label. This value will only be written if the label position does not correspond to a predefined label anchor. Also, this value will only be read if no label anchor is specified.	Yes	Yes
.y	attribute [Double]	The absolute y-coordinate of the label. This value will only be written if the label position does not correspond to a predefined label anchor. Also, this value will only be read if no label anchor is specified.	Yes	Yes
.borderDistance	attribute [Double]	The distance between label and the node's border.	Yes	Yes
.autoSizePolicy	attribute [String]	The auto sizing policy of the label. This is one of: "node_height", "node_width", or "content".	Yes	Yes
.fontSize	attribute [Integer]	The size of the label font.	Yes	Yes
.fontStyle	attribute [String]	The style of the label font, either "plain", "italic" or "bold".	Yes	Yes
.fontName	attribute [String]	The logical name of the label font, either "Dialog", "DialogInput", "Monospaced", "Serif", "SansSerif", or "Symbol".	Yes	Yes
.alignment	attribute [String]	The alignment of multi-line label, either "center", "left" or "right".	Yes	Yes
.rotationAngle	attribute [Double]	The label's rotation angle.	Yes	Yes
.icon	attribute [String]	URL for the icon's image.	Yes	No
.horizontalTextPosition	attribute [String]	A label's horizontal text positioning. This is one of: "left", "left_aligned", "center", "right", or "right_aligned".	Yes	No
.verticalTextPosition	attribute [String]	A label's vertical text positioning. This is one of: "top", "center", or "bottom".	Yes	No
.iconTextGap	attribute [Integer]	Distance between icon and label text.	Yes	No
.underlineText	attribute [Integer / Boolean]	Whether the label's text is underlined.	Yes	Yes
.visible	attribute [Integer / Boolean]	Whether the label is visible.	Yes	Yes

Table 9.10. GML .graph.edge level

Item	Type	Description	Parsed	Written
.source	attribute [Integer]	The ID of the source node (.graph.node.id).	Yes	Yes
.target	attribute [Integer]	The ID of the target node (.graph.node.id).	Yes	Yes
.label	attribute [String]	The textual name of the edge (label).	Yes	Yes
.graphics	section (single)	The graphical representation of the edge.	Yes	Yes
.edgeAnchor	section (single)	The port definitions.	Yes	Yes
.LabelGraphics	section (multiple)	Label graphics definitions.	Yes	Yes

Table 9.11. GML .graph.edge.graphics level

Item	Type	Description	Parsed	Written
.type	attribute [String]	The name of the linetype. This is one of: "arc", "spline", "bezier", or "quadCurve".	Yes	Yes
.quadCurveStraightness	attribute [Double]	The straightness of a quad curve edge.	Yes	Yes
.customconfiguration	attribute [String]	The configuration of a generic edge.	Yes	Yes
.userdataclass	attribute [String]	The class name for the user data part of a generic edge.	Yes	Yes
.userdata	attribute [String]	The user data part of a generic edge.	Yes	Yes
.smoothBends	attribute [Integer / Boolean]	Whether the edge has smoothed bends.	Yes	Yes
.width	attribute [Integer]	The stroke width used for the line.	Yes	Yes
.style	attribute [String]	The style used for the line. This is one of: "line", "dashed", "dotted".	Yes	Yes
.fill	attribute [String]	The color of the line.	Yes	Yes
.arrow	attribute [String]	"last", "first", or "both" depending on whether there are arrowheads at the edge's source end and target end.	Yes	No
.sourceArrow	attribute [String]	The type of the arrowhead at the edge's source end. This is one of: "delta", "standard", "diamond", "short", "white_delta", "white_diamond", or "none". If the arrowhead is of custom type, its name is encoded.	Yes	Yes
.targetArrow	attribute [String]	The type of the arrowhead at the edge's target end. This is one of: "delta", "standard", "diamond", "short", "white_delta", "white_diamond", or "none". If the arrowhead is of custom type, its name is encoded.	Yes	Yes
.reversedPathRendering	attribute [Integer / Boolean]	Whether the edge's path is rendered starting at the target end instead of the source end.	Yes	Yes
.arcType	attribute [String]	The type of an arc edge. This is one of: "fixedHeight" or "fixedRatio".	Yes	Yes
.arcHeight	attribute [Double]	The height of an arc edge.	Yes	Yes
.arcRatio	attribute [Double]	The ration of an arc edge.	Yes	Yes
.line	section (single)	The positions of the ports and bends of the edge.	Yes	Yes

Table 9.12. GML .graph.edge.graphics.line level

Item	Type	Description	Parsed	Written
.point	section (multiple)	A single bend or port.	Yes	Yes

Table 9.13. GML .graph.edge.graphics.line.point level

Item	Type	Description	Parsed	Written
.x	attribute [Double]	The x-coordinate of the point.	Yes	Yes
.y	attribute [Double]	The y-coordinate of the point.	Yes	Yes

Table 9.14. GML .graph.edge.LabelGraphics level

Item	Type	Description	Parsed	Written
.type	attribute [String]	The content type of the label, either "text" for textual labels or "index_label" for automatic numeric labels.	Yes	No
.text	attribute [String]	The label's text.	Yes	Yes
.outline	attribute [Color]	The border color of the label.	Yes	Yes
.fill	attribute [Color]	The background color of the label.	Yes	Yes
.color	attribute [Color]	The text color of the label.	Yes	Yes
.model	attribute [String]	The model of the label that determines how to interpret the position of a label. Known models are "center_slider", "side_slider", "six_pos", "three_center", "two_pos", "free" and "centered". For each model certain label positions can be assigned (see below).	Yes	Yes
.position	attribute [String]	The symbolic position of the label, that determines the actual position of the label. Known positions are "scentr", "center" "tcentr", "shead", "head" "thead", "stail", "tail" and "ttail". The available edge label models are "CENTERED", "TWO_POS", "THREE_CENTER", and "SIX_POS". "CENTERED" "TWO_POS" "THREE_CENTER" "SIX_POS"	Yes	Yes
.x	attribute [Double]	The absolute x-coordinate of the label. This value must be used to specify the position of the label for the label model types "center_slider", "side_slider" and "free".	Yes	Yes
.y	attribute [Double]	The absolute y-coordinate of the label. This value must be used to specify the position of the label for the label model types "center_slider", "side_slider" and "free". "CENTER_SLIDER" "SIDE_SLIDER" "FREE"	Yes	Yes
.fontSize	attribute [Integer]	The size of the label font.	Yes	Yes
.fontStyle	attribute [String]	The style of the label font, either "plain", "italic" or "bold".	Yes	Yes
.fontName	attribute [String]	The logical name of the label font, either "Dialog", "DialogInput", "Monospaced", "Serif", "SansSerif", or "Symbol".	Yes	Yes
.alignment	attribute [String]	The alignment of multi-line label, either "center", "left" or "right".	Yes	Yes
.rotationAngle	attribute [Integer]	The label's rotation angle.	Yes	Yes
.icon	attribute [String]	URL for the icon's image.	Yes	No
.horizontalTextPosition	attribute [String]	A label's horizontal text positioning. This is one of: "left", "left_aligned", "center", "right", or "right_aligned".	Yes	No
.verticalTextPosition	attribute [String]	A label's vertical text positioning. This is one of: "top", "center", or "bottom".	Yes	No
.iconTextGap	attribute [Integer]	Distance between icon and label text.	Yes	No
.underlineText	attribute [Integer / Boolean]	Whether the label's text is underlined.	Yes	Yes
.visible	attribute [Integer / Boolean]	Whether the label is visible.	Yes	Yes

Table 9.15. GML .graph.edge.edgeAnchor level

Item	Type	Description	Parsed	Written
.xSource	attribute [Double]	The position of the source port x-coordinate relative to half its width measured from the center of the node.	Yes	Yes
.ySource	attribute [Double]	The position of the source port y-coordinate relative to half its height measured from the center of the node.	Yes	Yes
.xTarget	attribute [Double]	The position of the target port x-coordinate relative to half its width measured from the center of the node.	Yes	Yes
.yTarget	attribute [Double]	The position of the target port x-coordinate relative to half its height measured from the center of the node.	Yes	Yes