label.java

asm的源码包 并且包含英文的文档

     * The next basic block in the basic block stack. This stack is used in the
     * main loop of the fix point algorithm used in the second step of the
     * control flow analysis algorithms.
     * 
     * @see MethodWriter#visitMaxs
     */
    Label next;

    // ------------------------------------------------------------------------
    // Constructor
    // ------------------------------------------------------------------------

    /**
     * Constructs a new label.
     */
    public Label() {
    }

    /**
     * Constructs a new label.
     * 
     * @param debug if this label is only used for debug attributes.
     */
    Label(final boolean debug) {
        this.status = debug ? DEBUG : 0;
    }

    // ------------------------------------------------------------------------
    // Methods to compute offsets and to manage forward references
    // ------------------------------------------------------------------------

    /**
     * Returns the offset corresponding to this label. This offset is computed
     * from the start of the method's bytecode. <i>This method is intended for
     * {@link Attribute} sub classes, and is normally not needed by class
     * generators or adapters.</i>
     * 
     * @return the offset corresponding to this label.
     * @throws IllegalStateException if this label is not resolved yet.
     */
    public int getOffset() {
        if ((status & RESOLVED) == 0) {
            throw new IllegalStateException("Label offset position has not been resolved yet");
        }
        return position;
    }

    /**
     * Puts a reference to this label in the bytecode of a method. If the
     * position of the label is known, the offset is computed and written
     * directly. Otherwise, a null offset is written and a new forward reference
     * is declared for this label.
     * 
     * @param owner the code writer that calls this method.
     * @param out the bytecode of the method.
     * @param source the position of first byte of the bytecode instruction that
     *        contains this label.
     * @param wideOffset <tt>true</tt> if the reference must be stored in 4
     *        bytes, or <tt>false</tt> if it must be stored with 2 bytes.
     * @throws IllegalArgumentException if this label has not been created by
     *         the given code writer.
     */
    void put(
        final MethodWriter owner,
        final ByteVector out,
        final int source,
        final boolean wideOffset)
    {
        if ((status & RESOLVED) != 0) {
            if (wideOffset) {
                out.putInt(position - source);
            } else {
                out.putShort(position - source);
            }
        } else {
            if (wideOffset) {
                addReference(-1 - source, out.length);
                out.putInt(-1);
            } else {
                addReference(source, out.length);
                out.putShort(-1);
            }
        }
    }

    /**
     * Adds a forward reference to this label. This method must be called only
     * for a true forward reference, i.e. only if this label is not resolved
     * yet. For backward references, the offset of the reference can be, and
     * must be, computed and stored directly.
     * 
     * @param sourcePosition the position of the referencing instruction. This
     *        position will be used to compute the offset of this forward
     *        reference.
     * @param referencePosition the position where the offset for this forward
     *        reference must be stored.
     */
    private void addReference(
        final int sourcePosition,
        final int referencePosition)
    {
        if (srcAndRefPositions == null) {
            srcAndRefPositions = new int[6];
        }
        if (referenceCount >= srcAndRefPositions.length) {
            int[] a = new int[srcAndRefPositions.length + 6];
            System.arraycopy(srcAndRefPositions,
                    0,
                    a,
                    0,
                    srcAndRefPositions.length);
            srcAndRefPositions = a;
        }
        srcAndRefPositions[referenceCount++] = sourcePosition;
        srcAndRefPositions[referenceCount++] = referencePosition;
    }

    /**
     * Resolves all forward references to this label. This method must be called
     * when this label is added to the bytecode of the method, i.e. when its
     * position becomes known. This method fills in the blanks that where left
     * in the bytecode by each forward reference previously added to this label.
     * 
     * @param owner the code writer that calls this method.
     * @param position the position of this label in the bytecode.
     * @param data the bytecode of the method.
     * @return <tt>true</tt> if a blank that was left for this label was to
     *         small to store the offset. In such a case the corresponding jump
     *         instruction is replaced with a pseudo instruction (using unused
     *         opcodes) using an unsigned two bytes offset. These pseudo
     *         instructions will need to be replaced with true instructions with
     *         wider offsets (4 bytes instead of 2). This is done in
     *         {@link MethodWriter#resizeInstructions}.
     * @throws IllegalArgumentException if this label has already been resolved,
     *         or if it has not been created by the given code writer.
     */
    boolean resolve(
        final MethodWriter owner,
        final int position,
        final byte[] data)
    {
        boolean needUpdate = false;
        this.status |= RESOLVED;
        this.position = position;
        int i = 0;
        while (i < referenceCount) {
            int source = srcAndRefPositions[i++];
            int reference = srcAndRefPositions[i++];
            int offset;
            if (source >= 0) {
                offset = position - source;
                if (offset < Short.MIN_VALUE || offset > Short.MAX_VALUE) {
                    /*
                     * changes the opcode of the jump instruction, in order to
                     * be able to find it later (see resizeInstructions in
                     * MethodWriter). These temporary opcodes are similar to
                     * jump instruction opcodes, except that the 2 bytes offset
                     * is unsigned (and can therefore represent values from 0 to
                     * 65535, which is sufficient since the size of a method is
                     * limited to 65535 bytes).
                     */
                    int opcode = data[reference - 1] & 0xFF;
                    if (opcode <= Opcodes.JSR) {
                        // changes IFEQ ... JSR to opcodes 202 to 217
                        data[reference - 1] = (byte) (opcode + 49);
                    } else {
                        // changes IFNULL and IFNONNULL to opcodes 218 and 219
                        data[reference - 1] = (byte) (opcode + 20);
                    }
                    needUpdate = true;
                }
                data[reference++] = (byte) (offset >>> 8);
                data[reference] = (byte) offset;
            } else {
                offset = position + source + 1;
                data[reference++] = (byte) (offset >>> 24);
                data[reference++] = (byte) (offset >>> 16);
                data[reference++] = (byte) (offset >>> 8);
                data[reference] = (byte) offset;
            }
        }
        return needUpdate;
    }

    /**
     * Returns the first label of the series to which this label belongs. For an
     * isolated label or for the first label in a series of successive labels,
     * this method returns the label itself. For other labels it returns the
     * first label of the series.
     * 
     * @return the first label of the series to which this label belongs.
     */
    Label getFirst() {
        return frame == null ? this : frame.owner;
    }

    // ------------------------------------------------------------------------
    // Overriden Object methods
    // ------------------------------------------------------------------------

    /**
     * Returns a string representation of this label.
     * 
     * @return a string representation of this label.
     */
    public String toString() {
        return "L" + System.identityHashCode(this);
    }
}